Half-Life 2 was originally featured in the original Game Music post, but only with a limited selection of the available (awesome) songs. Here is a more in-depth compilation of my favourite Half-Life 2 tracks, including tracks from Episodes 1 and 2.
Many of the Half-Life 2 tracks use extremely heavy baselines; they should be listened to on a decent sound system, with a decent sub woofer, in order to give the tracks, the justice they deserve :)
In addition to the note above; the quality of the clips below compares only marginally to that on the official sound track (which happens to come with Audiosurf).
Half Life 2
Triage at Dawn
Requiem for Ravenholm
Half Life 2 – Episodes 1 & 2