I know for a fact GoPro gets its music from extrememusic.com, and I personally use Night3x from youtube for electronic music, and RFGB for orchestral music.
AudioJungle offers some great services to.
Another source is CC-Mixter, straight from the creative commons, but I myself do not think that the library present is too good.
If you are looking for audio (eg. wind noise, etc.) then check out freesound.org, but check to make sure that you can use the track.
I've used AudioMicro.com and like a lot of their music. I hear good things about incompetech.com. You can also just Google "royalty-free music sites" and you'll find several, each with different terms and prices.
musicbed.com is probably the best quality I've heard from these types of sites. Of course, it's a little more expensive. But when you need quality, you should be prepared to pay for it. They have tons of good stuff already, but you can also actually commission them to write custom music for your project.
What is free music?
Free music is music that is not managed by performance rights organizations (PRS, ASCAP, SOCAN, BMI, BUMA, JASRAC...). Artists choose to protect their rights through specific non exclusive licenses such as Creative Commons. 'Free music' doesn't necessarily mean free-of-cost or devoid of any rights restrictions, it means "some rights reserved". Artists define what rights they grant on their music through their choice of license.
What is Creative Commons?
Creative Commons are licenses that enable musicians to give away their music for free while protecting their rights. They are easy to use and compatible with internet standards, and allow rights holders to authorize (or not) certain uses of their music, such as commercial uses and derived works.
Well there is a source I have used a lot in the past. Very popular, a lot of others have used as well. Its free, legal and good quality. but I tend to pay a token fee for stuff I use ($5) though there is no obligation to do so.
I've used Adrenalin Sounds before and they have a great selection of music and an easy search system. If you can't be bothered going through a heap of tracks they will create a playlist for you, which I have found helpful in the past.
From my experience there are some parameters you need to check for each of the sites mentioned here already:
What is their licensing model: usually the licensing depends on the distribution channel you use, i.e. "unlimited worldwide TV distribution" usually being the most expensive.
How does their "curatorial" skills meet your quality standards. I wouldnt bother browsing a site that is cheaper but their selection of music is mainly crap. There are huge quality differences of the selection they offer and usually the free sites have the worst "crap" vs "quality" ratio.
Check what types of music, license wise, the site is distibuting. In the case of extrememusic.com i know that not all of their tracks are royalty free. While the music they offered is great, you might find stuff that is actually registered with ascap, gema, whatever and have to negotiate royalties with them, which is often unacceptable given the timeframe in which music selection has to take place.
Consider hiring a composer: Sometimes it is faster, and not even more expensive to let somebody write a piece of music specifically tailored to your needs. I wouldnt rule that option out and that way you avoid using a music bed that may have been already used in a context you dont like.
Don't assume that if a music file is offered for free, that this holds true. Especially in sites that offer user maintained content, you can only be sure that the file is really royalty free if you got that in a written statement. Companies specializing in these kind of music take care of that, if using stuff from a "free" site i would contact the author of the track and get a written confirmation to avoid legal consequences.