As a freelancer, getting people to look or listen to your work is step 0. For sound design, there are a veritable crap load of websites out there, which complicates things! So then, which music website is right for you? Well, read the rest of this post and make an informed decision.
SOUNDCLOUDwww.soundcloud.comMy website of choice, and probably the most prominently used by the public. Post up your work, comment on other people's tracks like crazy, and you will have a following guaranteed. Sharing to groups is a great feature; this allows you to show your track directly to those interested in your genre or style. Upgrading your account is unnecessary and costs $$, but comes with some nice features, including nice music playing widgets for your website, more space, and more comprehensive tracking of listeners.
THE SIXTY ONEwww.thesixtyone.comIt's like an MMO, but instead of clicking on fantastic creatures for EXP, you listen to music for EXP! As a listener you get daily music listening "quests", level up, and gain more features. As an artist, it's super easy to get set up and discovered due to the way the site plays randomized tracks based on genre. Selling downloads is integrated into the site, which is great if you'd like to utilize that. Give this one a try, a good number of people have gotten recognition from this site. Keep in mind that the first $20 you make will go to the website.
MYSPACEwww.myspace.comI think most of this company is no longer operating, so... choose this if you're feeling strangely eclectic.
INDABAwww.indabamusic.comThis site markets itself as a complete solution for musicians. It's very nicely designed and uploading is a breeze. They also do their own licensing and contracting, so you can get your foot in the sound industry's door pretty quickly if you play your cards right. If you start using this heavily, expect to pay $$ to upgrade your account, as a free account is pretty limited.
BEATPORTwww.beatport.comHUGE for electronic music and DJs. Has a section where you can upload packs of loops and SFX, which is pretty dandy. Not too applicable towards video game music, though..
8BITCOLLECTIVEhttp://8bc.orgIt's down at the moment, but this has always been a great place to post chiptunes and chiptune-inspired stuff. The community was active and there were no $$ required. Hopefully this comes back soon!
BANDCAMPwww.bandcamp.comA very easy to use storefront for independent musicians. But that's all it is: a storefront. If you just made the OST to a well selling game, you can put it up on bandcamp and get a lot of purchases. If you're not well known, you must advertise yourself somehow. RoeTaKa expands on this:
If you think "my album is killer, when it goes on bandcamp I'll see a cash flow!" that is pretty wishful thinking without hype and awareness of it. People usually try to big up their releases with previews, some sort of (internet)radio play and other means...it's easier for bands that can make gigs and properly promote their releases. Danny B obviously can swear by Bandcamp because Super Meat Boy was so successful and people knew the soundtrack was available.
JAMENDOwww.jamendo.comAnother website that allows people to browse and license tracks you put up. I've not heard much about this one, but users here have made $$ from it.
PUMP AUDIOwww.pumpaudio.comA website that can get you licensed insanely quickly with its huge client and user base. Their acceptance process can take 6 months to a year.
I'm sure I missed a bunch, so let's discuss. I'll be updating this post as new sites are posted.