I remember one game I had for PC which was Praetorians, a RTS games set in Roman times. The music was nice but whenever a fight started it would quickly change and pump out some Rob Dougan (Usually if not something just as awesome) and his songs made the fights so epic. Not the game's music but it worked so well for that game.
EDIT: as for original game music I would have to say HL2 had a lot of good ones. I have them on CD and listen to them today, they have good action, peaceful, scary, creepy, tense and many other types of songs and they all seem to fit perfectly.