Steam music and free soundtracks

Miner Wars 2081 :D

Audiosurf give you music from portal 1 and 2, half life (all chapters) and team fortress

Finding Teddy comes with a Soundtrack, as far as I know. Also on the game 'A Virus Named Tom' I see soundtrack DLC, but don't have the game downloaded so I can't confirm.

Related note: I remember The Binding of Isaac soundtrack being on steam, why they removed it?

Finding Teddy, has an Soundtrack DLC you get with the game but it is not in the Steam Music Player. As for games with soundtracks for the Steam Music Player, I can add Obscure and Element4I

Company of Heroes
Greed: Black Border
Warhammer® 40,000™: Dawn of War® II (direct link, can't find it on Store page)

If you install Miami Hotline, you have access to OGG files of the whole soundtrack in its folder.
Worth listen it if you like electronic music ;)

Rock of Ages,
GTR Evolution (it's just one music),
Afterfall: InSanity Extended Edition (+ Afterfall: InSanity - Music Inspired by the Game)

As far as I know, if a game has a soundtrack and it's downloaded it will appear in the music player.

If you have unlimited bandwidth, just download all the games to determine which come with soundtracks!

Sparkle 2 Evo (it appears just as 'OST' on the albums list)