I wrote about music for RPGs before and the post seems fairly popular, so here's another one.
My starting point is this article at Gnomestew reviewing a long list of different soundtracks for RPGs. This one about creating playlists is definitely worth reading as well.
Obsidian Portal has a great site with playlists and a list of links to web radio stations and free music for RPGs.
Among those is Two Steps from Hell Radio.
Two Steps from Hell have produced the music for a lot of trailers and it's awesome for action sequences in particular.
Sounds to sleep to offers a couple of ambient noises like campfire of chirping crickets.
Radio Rivendell plays, surprise, music for fantasy RPGs.
Dayjo.org offers a couple of free music pieces that are very cool for scenes where you need just some ambient music, nothing too dramatic or suspenseful.
And finally, my personal favourite:
Incompetech offers a massive collection of music free to listen to and download. You can search for music to create a certain mood or you can just take a look at the different collections of film scores (scroll right down to the bottom). There's also a lot of other music there, from folk to electronica.
So if you'll excuse me, I'm off to browse Incompetech. I need to get my stuff together for the start of my Endland campaign in four weeks.