There are lots of royalty free music sites out there to submit your music to but there are more and more sites appearing that actually pitch your music to certain projects and producers for free or for a fee. There are also a few crowd-sourcing / collaboration type sites here.
These sites are distinctly different from the free-for-all type sites like productiontrax, audiojungle and audiosparx and might be an interesting addition to your search for outlets for your production music.

1. Humtoo.com
http://www.humtoo.com/

A global meeting place for music makers and content creators. This site is pretty cool as it is free to signup and submit your music to videos and project descriptions.

2. Film Music.net
http://www.filmmusic.net/
Paid monthly subscription to submit music to be considered

  Immediate Access to our Film/TV Music Jobs Database
Access to Film Music Network Live! 24 hr/day streaming audio of Film Music Network events
Immediate Access to our Film & TV Music Salary & Rate Survey
Full benefits and discounts at all participating Film Music Network Vendor Partners.

3. The Composer Collective
http://www.thecomposercollective.com/tcc/composersignup.asp

The Composer Collective is comprised of cinematic composers dedicated to creating dramatic scores of the highest quality for film, television, interactive and commercial media. We group composers into innovative and productive workforces, giving the film industry a much-needed resource for intelligent music at never-before-seen productivity levels. This service is known as TeamScore™.

Choosing TeamScore™ puts you on the front lines of the film business as it changes and adapts rapidly to the demands of distributors and consumers worldwide.

4. Taxi.net
https://www.taxi.com/join/?88.7.207.46.6651122970743537
Paid yearly subscription is $299.95

http://www.taxi.com/abouts/join-whatyoullget.html
More than 1,200 opportunities per year to pitch your music to Major and Indie Record Labels, Top Music Publishers, and Music Supervisors working on film and TV projects. That’s TEN TIMES the number of opportunities you’ll get with TAXI imitators! And don’t forget, we have opportunities in nearly every conceivable genre of music.

5. Broadjam
http://www.broadjam.com/delivery/index.php?sessionID=q4qqrqkt2vc05bi4s15palahj1
offers an ‘opportunities’ service for $5-$10 per sbmission depending on what type a paid account you have

a broadjam member to enter opportunities. sign up today. you’ll also get other services designed to help independent musicians like you, promote themselves.

6. Studentfilms.com Forum
http://forums.studentfilms.com/eve/forums/a/frm/f/6156029451

Jobs posted for composers and composer submitting their resumes and profiles. Free to register.
Lots of posts like ‘Composer Needed’ and ‘Composer Available’.

7. Soundreef (beta)
http://www.soundreef.com/private_beta/promoterPage?o=13

Swap music for promotion? Not sure what that means but worth investigating.

8. YouLicense
direct job opportunities are available here
http://www.youlicense.com/Opportunities/default.aspx

here’s the RSS feed for the jobs list 
http://www.youlicense.com/XML/RSS/Opportunities/YouLicenseOpportunities.xml

9. Minimum Noise
http://www.minimumnoise.com/
A new site with all kinds of projects for "crowdsourcing musicians".

10. Pump Audio
http://pumpaudio.com/artists/index.php

There is no submission fee. If your music is not used, you lose nothing.
You will receive 50% of the license fees we receive for your music.
Pump’s deal is completely non-exclusive.
Our deal will never prevent you from working with anyone else.

11. Musicdealers.com
http://musicdealers.com/
A new site where you can upload and create a profile and the people at musicdealers pitch your tracks to potential clients.

12. Musicloops.com
Our site where you can submit a demo. No fees to join. Price your own tracks, lots of sales, with 50% of the price goes to the composer.

Some great questions from musicformedia over at filmandgamecomposers.com:

When you first created your library was it all music you had already made, or was it stuff that you created specifically for the purpose of selling in a stock music library?

We started out our music career as composers for TV shows and commercials, radio ads and video games. We would always give our clients 3 or 4 different ideas to choose from for their spot. These were all fairly well-developed ideas. They would choose one and the rest would go on the shelf. Our first collection of royalty free music (published in 1996) was a collection of these alternate choices.

Once that collection started selling we realized we needed to create music specifically for our production music library.

If you were creating your library of stock music from scratch again, what would you do differently?

I think I would have kept track of the different mixes better. In the old days once a mix was done the set up was pretty much lost forever. Now we can recall any mix and have it come back sounding exactly the way it did a couple of years ago.

Do you think there is a set “package” of types of music you should upload? What I mean by this is, if you sell a lot of music, is there a certain amount of of types that sell more than others – ie. should you create a library of 50 songs (each with a 60 second edit, 30 second edit, 15 second edit and 2 or 3 loops), maybe 4-5 sound effect bundles – like a “Video Game” Sound Effect Bundle, “Horror Movie” Sound Effect Bundle etc. I hope this makes sense – my general question is, should you be creating a set amount of each type to maximise sales?

Bundles are great, the more creative the better. In my experience many of our customers go for the full length track but people who just need a loop for their website will buy one or two of these from the package. Lots of people buy the 60 second version because it is usually a bit cheaper.

We have some composers who upload bundles of music loops and corresponding button sounds. Music and complimenting sound effects is a good idea (we did this with our Horror! collection and it sells very well).
In your case maybe some nice ambient nature sounds to go along with your piano music.

Some advice on pricing your packages; if your full length track is 1:30 I would price it the same as your 60 second version.
In general I would price the 60 second version of the tracks close to or the same as the full length track price.

How long are your tracks usually? I have a lot of 20-30 second piano pieces, but I’m not sure they’re long enough.

This is considered fairly short, you might want to extend them. 20 seconds is good for a website, most of our 20 second loops go for $9.95.
The 30 second version might be $14.95 or $19.95 but you really want to get up to at least 60 seconds for most uses.

I’ve noticed a few full sized scores for films – ie. 10-15mins tracks – do you sell any of these yourself, and do you find they sell well?

Most full scores are actual symphony recordings of classical music. At least on our websites I haven’t seen any composers uploading anything over 5 or 6 minutes.

I have been selling royalty free music for more than 10 years now. I receive composer submissions and demos on a daily basis. I also see what sells and what customers are requesting everyday.
I don’t often give advise to composers but thought I would post some basic observations in case it is of any use to composers looking to get into the royalty free music industry.

1. Be Prolific
If you want to make a living at selling your production music the first thing you need is a large library of music.
The composers who have large catalogs on our sites earn the most and earn very consistently. 75-300 tracks and up is considered a large library.

2. Create edits and loops with your full length tracks.
The customers on our websites love the fact that we offer not only full length tracks but edits, loops, underscores and alternate takes. You can also sell the whole package of full length track, edits for a higher price than the full length track alone.
This coupled with a large library will practically guarantee steady sales (as long as the music is top quality of course).

A typical package would be:
Full length track 2 to 3 minutes
60 second edit
30 second edit
15 second edit
2 or 3 loops.

3. Write What You Know
Professional production music composers are a very versatile lot. They can write music in many styles and can do it convincingly. However many of the demos I receive are from composers who are trying to be all things to everybody, writing in as many styles as possible with the majority of the track coming across as mediocre. We pass on these types of demos 99% of the time.
If you are an excellent New Age Music composer don’t try your hand at Nu-Metal just to fill out your catalog, write more New Age Music instead. Write the music you love to write, not what you think will sell.

4. Add Something Real
In this day of laptop studios, garageband and reason it seems that everyone is now trying their hat at being a “composer”. I may be old skool but back in the day musicians used to practice their instruments for hours everyday and went to school to study harmony and composition. Today it seems that anyone who can download some free drum loops from the web calls themselves a composer.
All this to say when you compose try adding something real to your compositions. Instead of only relying on your loop library (the one that thousands of other people are using) try adding some real guitar parts, or a weird vocal part, or a sax, get your friend to play harmonica, pick up any simple percussion instrument, even a tambourine, and record it live. Give your standard loop library composition a soul by adding something real.

5. Melody
Just like in popular music melody plays a big factor in royalty free music. Sure there are lots of instances where you want the music to sit in the background and not attract too much attention but according to our sales stats music with a good, strong, uplifting melody outsells all other types of music.

6. Holiday Music
Would you like a Christmas bonus every year in your royalty free music paycheck? Then do Christmas and holiday music in whatever style your composition skills are strongest. A hip hop version of Jingle Bells, an ozzy osbourne version of The First Noel.
This would go for all types of public domain music, a Nu-Metal version of God Bless America, a jazz version of the Star Spangled Banner, drum n bass Auld Lang Syne, etc.
Our customers just love this kind of stuff.

7. Structure
Make sure to think about the listener when you’re are putting together the structure of your royalty free music tracks.

a. Don’t have a 2 minute intro before getting to the main melody. You need to grab the listener quickly, get to the point as soon as possible (within reason of course).

b. Give them an ending. No fades. Give them a proper ending with a chord and cymbals that ring out. This is very important for the ends of commercials and radio spots.

c. Give them a B section, also known as a bridge. You can do the same thing with a breakdown if it is dance music. You need to give the customer some variety in the track, something to play with in editing.

Hope this helps. Let me know what you think.
I’ll be posting more tips for royalty free music composers in the coming weeks.
-Mark

Here’s a great question that I found on the Film and Game Composer’s Forum from a member who is a roylaty free music composer:

"Does anyone have any kind of knowledge about keywords and descriptions on royalty free sites? Does it REALLY help sales? I mean considering the extra time required to think up and input these terms…"

I believe I have extensive knowledge in this area and yes, there is no doubt that good descriptions and keywords for your tracks have a big impact on sales.

"If it is a helpful tool, then what are some of the more popular techniques that are generally employed by composers and producers on Royalty Free websites…"

Here’s a list of general concepts that will help you to fill out your descriptions. If you answer most or all of these questions in your description plus add a little flare and creativity you will end up with a great description that compels the customer click on the play preview button.

Things to convey to the customer
what genre(s) it is:  jazzy, fusion, latin, rock, speed metal
what tempo: uptempo, slow,
type of feel: funky, laid back
instruments used with descriptive adjectives: real guitar, swirling synths, majestic strings
emotion: sad, lonely, happy, soulful, lost,
what type of use: children’s show, wedding video, corporate presentation, hollywood blockbuster
structure: is there a bridge, is there a chorus, is there a breakdown, etc?
sounds like?: name some bands that the music might be similar to.

Examples:

Michele Vanni writes great descriptions

Rearview
The road lies ahead, a new adventure, and in the rearview mirror we see familiar
places fading in the distance. The radio plays this happy and somewhat nostalgic
tune driven by 12 string acoustic guitars. The main theme is played with more energy
and additional electric guitar the second time around.

Dan Morrissey also writes great descriptions

Exit : Stage Front
Twisted synth sequences get this pitbull of a track moving, then massive guitar riffs
crash in with drums and bass to form an irrrestistable groove. Imagine a cross between
White Zombie and Kraftwerk. Contains drums, synth ,bass and lots of guitars!

Bjorn Lynne also knows the value of a great description,

Spellcraft
Depicting a magic fantasy world of fairies and deep forests full of magical creatures and
ancient adventures. Otherworldly, pastoral. Piano, vocal pads, tremolo guitars, mysterious
sounds and hand drums.

Customers love descriptions! It makes it easier for them to browse and gets them interested in the tracks before they even hear it. Remember that customers more often than not use the search function of royalty free music websites. If your description says "rock song" it will probably never be found or purchased.

With increased competition in the royalty free music marketplace there are lots of sites popping up that are selling music for ridiculously low fees. They are asking composers to upload their tracks promising them only 40% or 45% of a $10.00 fee. The composer would take home whopping $4.50 for a high quality full length track.
$4.50 will barely get you a gallon of gas.
These sites do not offer any backend royalties either.

I urge all royalty free music composers to avoid these types of sites or at least give some serious thought to what you are doing before you upload. These sites will only devalue your catalog and make it hard for other distribution channels to accept your music. If your track is on sale for $10.00 on eCheapMusic.com then it will make it very difficult for a proper site to justify charging a higher price for your music elsewhere.

Just my 2 cents but I know that many other royalty free music site owners agree with me on this one.