Quest for Glory 5 - this was the soundtrack that finally got gaming music added as an award category.

Which sort of tracks you'll like depends on what sort of music you like though. For example, Tropico 3 has a wonderful soundtrack for the game's theme but it's a bit outside the usual and if you don't like the style then you won't like the music.

With that caveat in mind, I'll add these two to everything else that's been suggested:

I usually list individual tracks rather than entire soundtracks, but the Elder Scrolls games (Morrowind, Oblivion, Skyrim) are a great set.

The main theme (okay, it's the only theme) from Legend of Grimrock is also very well done.

Edit:

Among older (sometimes very old) games that I'm familiar with...

The Ultima series. "Stones" is by far the most iconic piece from this series, but the Ultima Underworlds, Ultima 8 and Ultima 9 all had some great pieces. (U6 and the U7s are generally too short to make for good listening.)

Master of Orion II, as mentioned further down.

The "Age of Wonders" games.

"Lords of Magic". The game itself was really weak, but the soundtrack was great, especially the Order theme.

(I find that turn-based strategy and older RPG games - which are usually either turn-based or slow-paced - tend to have the best offerings, probably because they don't have the problem of needing to keep up with the presumed action of fast-moving game styles.)

the Quake soundtrack was written and performed by NIN /thread

Mace Griffin Bounty Hunter

"Stones" from the Ultima games.

Unreal Tournament OST is one of my favorite game OSTs.

Anything game from the Heroes of Might and Magic series. Most, if not all of the Might and Magic series will do in a pinch as well :p

Go play World of Warcraft. :) Isolate yourself from surrounding sounds, and enjoy this musical piece, inspired by the sad events that took place during patch 5.1 and the Purge of Dalaran.

http://www.youtube.com/watch?v=hldpIOkwusg

I am sad. No one has posted Hell March

Age of Conan: Memories of Chimmeria... never say PC gaming does not have good music!

http://www.youtube.com/watch?v=deDNzOYHu-4

I'd second CIV4 too but will never forget the old MoO2 soundtrack either.

is this in-game music, or the odd track we listen to when gaming?
if the first i find many of the tracks in halo 1 and 2 fun to game to and add a lot to the experience. i remember when i changed my speakers to a 2.1 system in halo 2 and i was blown away at how atmospheric a particular level was! (in this case halo 2, but i do sometimes play halo 3, and halo reach music when i game to halo).
Red Alert's music tracks were awesome too. trying to think of any others that stick in my mind.

if the latter, i love (for some odd reason) playing unreal tournament 2K4 to Pink - Sober. for some odd reason my stats always greatly improve in that match when i listen to that track!

It's not so much about the music itself as:
1. 8/16 bit era soundtracks had technological limitations which endorsed the use of simple and memorizable melodies (much simpler than the stuff you had on Amiga and in DOS games at the time). And if a piece is easy to remember AND you like the context you heard it in you will love it out of nostalgia.
2. There's something about those 80's/90's console games that generally brainwashes its fans to believe that they are the most amazing stuff in every possible way - including the music. :P

Anyway, some "PC" game music I loved (although some of them are multi-platform - also the list would normally be longer but people have already suggested some great stuff :P):
Hitman - Codename 47: Jungle Exploration
Hitman - Codename 47: Harbor Themes
Command & Conquer (Full Soundtrack)
Warcraft 2 - Tides of Darkness (Full Soundtrack Playlist)
StarCraft (Full Soundtrack)
Freedom Fighters (Full Soundtrack Playlist)
Dawn of War - Space Marines Theme
Max Payne Main Theme
Mafia (Full Soundtrack Playlist)

Also some awesome Amiga soundtracks:
Turrican II - Title Theme
Turrican (Full Soundtrack Playlist)
Shadow of the Beast - Intro + Level 1
Chaos Engine - Main Theme

A shame that all the music I'm thinking of is obscura.

Princess Maker II: Battle

Tyran 2000: Tyran, the Level.

Skyroads: Road I.

SimCity 3000: All of the music.

This one doesn't actually count because it doesn't actually play in the game, but: Noctis.mid. Also, I want to give a shoutout to this minimalistic universe exploration game.

Because the power of Riff AM compels you: Jazz Jackrabbit ][ Boss theme!

The music from the Sims always reminded me of this; it's a pretty clear influence.

http://www.youtube.com/watch?v=f2u0BvyDxdM