I’ve seen a lot of the good ones already listed. Here is a few I haven’t seen listed yet…
Outlaws
Clive Barker’s Undying
System Shock 2
Lands of Lore; Gaurdians of Destiny
I’m not sure if they can be found on you tube since I hardly ever use it.
Half Life series - Vortal Combat and DA Guitar
DODS - Main Theme Music
Red Alert 3 Uprising - Yuriko’s Tale, Soviet March 2 (soprano version).
Modern Warfare 2 - https://www.youtube.com/watch?v=s1prGfsxSVM (This one really made me emotional, seeing DC like that in ruins made me realize all that shit can really happen)
GTA SA:
https://www.youtube.com/watch?v=INebOvgPoTk
GTA VC:
https://www.youtube.com/watch?v=5p1vacn3KcE
Half Life: Nuclear Mission Jam:
https://www.youtube.com/watch?v=MpNXztu9ndM
Max Payne 1:
https://www.youtube.com/watch?v=gRuReVCFOY0
The rest of all the Half Life songs.
And the DOOM songs too…
Keep the thread alive people! ;p
Wow, Modern warfare 2 track
https://www.youtube.com/watch?v=T8KxfWdYIEM
Makes really emotional moment, listen to it EPIC song.
Theme song from MGS3 (the best MG game) https://www.youtube.com/watch?v=i_zIKKOjC7Q
also some SNES side-scroller game where you play as a cyborg and throw a boomerang at enemies, it had amazingly intense music.
Ahhh videogame music… where to start?
- Any Metal Gear Solid music (especially the main theme for MGS:2 and the Snakeboarding theme)
- Any Diablo music (mainly Diablo 1, but 2 and LoD are pretty good)
- The Halo OSTs (especially Heretic Hero and the Main Theme
- Spyhunter themes - the arcade version and the Playstation 2 version
- All the music from Half-Life 1, especially the VALVe theme and the end credits
- All Metroid music (especially Metroid Prime 2 and Super Metroid)
If I think of more I’ll update this list or add a post.
Martin O’Donnell and Michael Salvatori crafted some absolutely fantastic music for Halo, there is no doubt about that. Really atmospheric, ambient, not sure how else to put it. It puts you places 
Fuck yeah!
I’m surprised the midnight riders haven’t been suggested.
Also the music for dead rising while fighting Adam the clown:
https://www.youtube.com/watch?v=LhiS5EMM7MY
Age of Empires 2 famous theme music:
https://www.youtube.com/watch?v=MykR_ZNGXyE&feature=related
I completely forgot that Age of Empires1 theme was different:
https://www.youtube.com/watch?v=Ygppczv6VJo
This brings up so many memories! my first RTS ever…
I absolutely loved the music from Braid. Such a fantastic game. Couldn’t understand all of it, but it was great. 
I love all of the music from Braid, but I love Downstream to bits. :3