Not so much a soundtrack: The intro song for Skate or Die on the old Tandy 1000 3 voice chipset. For some reason that song caught my ear back then. Also the Tandy 3 voice version of Alice Cooper's School's out in one of the skating courses. Ah memories. Ah being old.

someone mentioned Unreal Tournament earlier, and it should definitely get an honorable mention, especially since it got me into trying out modplug/tracker, and for the time it got me back into making music. A few more:
Mechwarrior 2: Mercenaries (still probably my favorite soundtracks)
Riven
Donkey Kong Country 1
Final Fantasy VII/VIII
Tetris
F-Zero
Castlevania (Dracula X offhand)
..and dangit, lost my track of thought.

Metal Gear Solid Theme - beautiful, emotional
Portal - "Still alive" - Great song , "Maybe Black Mesa, that was a joke, ha-ha, fat chance..."
Some tracks from Morrowind soundtrack.

if you're referring to Gladius (I think) - Still Alive from Rock Band... how can you even remotely enjoy that song? It's ridiculously annoying...

You never played Portal. Shame on you.

No no, it's "bravo!" for not conforming to the masses. ;)
Plus I stopped playing potentially addictive games because they had a tendency to ruin what was left my life.

For game sound bites, "Groovy!" from Earthworm Jim.
For game songs I have to second "Still Alive" from Portal.
For game scores, the one that has really stuck in my head is the music from Advent Rising, yet another gem from Tommy Tallarico.

Portal is maybe 3 hours length, don't worry :)

Ice Wind Dale, I think it was a sarcastic voice pack but one of them would say 'Im ready to adventurin... n stuff'. I still blurt it out whenever I pick up an rpg :)
Also, pretty much anything from Starcraft and Duke; "
"Im about to drop the hammer and dispense some indiscriminate justice!"
and
"Its Time to kick ass and chew bubblegum... and Im all outta gum"

I would have to say the main theme from Max Payne and Max Payne 2. I can still hear those beautiful strings and chords echoing somewhere in my head.
- "Still Alive" (obviously)
- Any time that I play a Final Fantasy title, the music is stuck in my head for weeks.
- The soundtracks for all of the Silent Hill games (besides the shitty new one)
- The music from Fallout. And the NPC voices. "What the bloody bloody bloody hell do you think you're doing!?"

- "Still Alive" - I just LOVE that song :)
- Soundtrack from BG2 and Fallout

Morrowind, definitely. Oblivion main theme is rather good, but Morrowind is still better.
The Unreal Tournament theme is also a good piece, and I like the remix on it in UTIII.
Hell March for C&C is also a classic as well as the Monkey Island theme.
Recently the theme from upcoming Mirror's Edge has been stuck in my head. It's just so moody and, well, simply beautiful.
Other than that I do listen a lot to remixes of old classics from the C64 and Amiga such as Cybernoid II, Monty On the Run (highscore), Commando, Giana Sisters, Last Ninja... and the list goes on.

SoulBringer -- the music from this game stays with me til this day and really set the mood well for this delightful game.

1/ Totally agree
2/ WHAT?! The remix is good?!