These are the gamez I played and sometimes still play online:

• Quake 3 Arena
• Half Life - Counterstrike / DayOfDefeat / FireArms
• Delta Force (1, 2 and LW)
• Starsiege Tribes / Tribes 2
• Utopia

And of course there are many more which I am not playing online (Single player or only on LANs), like Age Of Empires 2, Atomic Bomberman, Soldier Of Fortune, ...