I was linked to this site the other day, a collection of remixes of System Shock music (as well as remastered mp3s of midis), and listening to the SS1 intro hit me with such a strong bout of nostalgia that I started reinstalling the game right then and there. Shadowrun (sega version)'s main theme has the same effect, as does Deus Ex: IW's, and basically anything associated with Jesper Kyd (I have the OSTs to all four Hitman games + Freedom Fighters on my MP3 player).
What gaming music (or imagery, or references) has this effect on you?