Good games I've played for PC
Halo; If the system supports it. Very fun MP.

if you don't get it for the single player, get it for the mp!
FreeSpace 2; Great game that kept me guessing til the end! Also many mods for it.
FreeSpace: The Great War; Another game that had me on the seat of my pants. Old, but good,
HardWar; Great game from about '98. Think Freelancer only with a much better storyline™.
Age Of Empires 2:The Conquerors; Old now, but still fun at times in my book.
Quake2; Just in case you just have the urge to just kill. MP I found was very annoying since it's plagued with bots and cheaters.
BattleZone 1 or 2; Great if you want a combo RTS and FPS.