-Free Soundtracks if you already own these games-

Valve Games Soundtracks
http://store.steampowered.com/search/?snr=1_5_9__12&term=soundtrack#term=soundtrack%20valve&sort_by=_ASC&category1=21&page=1

Half-Life 2 Soundtrack
Half-Life Soundtrack
Free to Play Soundtrack
Portal 2 Soundtrack
Portal Soundtrack
Half-Life 2: Episode One Soundtrack
Half Life 2: Episode Two Soundtrack

Btw if you are looking for the mp3s they will be under Steam\SteamApps\common folder
ex: C:\Program Files\Steam\SteamApps\common\Portal 2\Soundtrack