Don't have played the game yet, but the main theme I've heard is incredible (downloadable on composer's website).
By the way, in case some don't know this site, it has a great collection of old LucasArts soundtracks to download, with great cover art.
LucasArts have already asked them, some years ago, to not distribute Indy and Star Wars soundtracks. But now, as the old LucasArts classics are back on the scene, it might be possible that all the other soundtracks will disappear as well...
Go get them before it happens ;)

In no particular order:
Braid
Spellforce series
The Witcher
The Guild series (some great tunes there)
Planescape Torment
Rome: Total War
Arcanum
Total Annihilation
Max Payne 2
I'm sure I'm forgetting some, but that will have to do. :-)

Few of my favourites in random order:
Fallout 1&2
Homeworld series
Outcast
Prince of Persia trilogy (Sands, Warrior & Two Thrones)
World in Conflict
Heroes 3
GTA Vice City
Myst series
Resident Evil 2 & 3
Burnout Paradise
Mafia

Republic the revolution :)
Just bought the game solely for soundtrack as i will not be able to play the game for at least next three weeks.

Random favorites:
MechWarrior 2 series
Command & Conquer series (or anything else Frank Klepacki has done)
The Longest Journey
Starcraft
World of Goo
Rez
Silent Hill soundtracks for when I want to feel uncomfortable.

I'm a sucker for choruses, so I liked the intro song for Hitman 2 and ... there was this game written by Orson Scott Card that had plenty of choral music in it and the characters had long legs... I think it was Advent Rising though I've never played it before, but the music I found of it was great. Also a couple or Blizzard games have incorporated choral music in their intros or trailers.
Other than that, music is a very "in the background" thing for me.

A couple of my favorites were listed, but there's plenty still unmentioned, including Legend of Mana, Prince of Persia Sands of Time and Divine Divinity. There's a few more, but I can't think of them right now

Bump to show we have this thread already.

Since a thread like this has been bumped I guess I'll have to give props to the Unreal soundtracks, they're just so great and fit the games perfectly. My favourite tracks can be found in Unreal and the first Unreal Tournament, although UT2004 is has some pretty awesome music as well.

Battlefield 1942.
http://www.youtube.com/watch?v=vg0Tmydj29M
And Dragon Age of course.
http://www.youtube.com/watch?v=5h64wglmemQ&feature=related

Lots of games have me thinking "Oh, that's cool" over their soundtrack, and some have one standout track that sticks with you, but only a few games come to mind as having particularly memorable, consistently awesome soundtracks.
Thunderscape has possibly my overall favorite soundtrack. It's a little repetitive, but awesome from end to end. My favorite track.
A few others in no particular order:
Crusader: No Remorse + No Regret
Deus Ex
Ragnarok Online (, [url=http://www.youtube.com/watch?v=TDngwvUPk-0]seriously)
Warcraft 2
Final Fantasy IV
Prince of Persia: The Sands of Time

i don't think any soundtrack made me cream my pants quite like the monkey island 3 soundtrack! The midi-soundtracks from the previous games were also delicious. Otherwise the starcraft soundtrack managed to use incredibly many cheesy sounds, synth leads and guitars with chorus effect, and it still managed to maintain awesomeness!

The Last Ninja 1-3 [C64]
Chris H�lsbeck's ChipWar [C64]
Hubbard's Giana Sisters, Auf Wiedersehen Monty, Nemesis The Warlock, Zoids, One Man and His Droid [C64]
Total Annihilation
Neverhood
Descent 1+2, especially Allister's Brimble's PSX soundtrack [i believe it was for the PSX]
Shadowgrounds
Bionic Commando Rearmed ;P
Diablo1+2
Jets 'N' Guns
Dune2/C&Cs/RAs/Dune2k/Emperor:BfD
Doom1+2
Both Crusaders.

Still waiting on Mark Morgan's remastered Fallout soundrack to be released...
It was apparently available at one point, but now it's not available for download, and you can't even listen to the tracks online.
http://auralnetwork.com/releases

It's available on nma, bigdowloads, fileshack, etc. Let me know if you need a link.