Carmagedon 2 - It had some nice metal songs, really getiing you in the mood for killing innocent bystanders for fun.
Unreal Tournament and 2004, and 3 - All have some exellent music, and some of the only ones i listen to outside games.
Sauerbraten - No one of you probably heard of the game, and open source fps. The game is pretty good, but the music itself is even better.
Defcon - Hands down my favorite game soundtrack. Its just amizingly wellmade, and really draws you in.
Age of Empires and 2 - Loved those midi vibes.
Red alert - Hellmarch takes the 2nd place for me.
Portal - Sure the end credit song was good, but for me, its the ambient music during the stages, that takes the cake.
Stronghold Crusader - Had some nice background music too, if slightly understated.