I like online games against other people, but mini online games don't really interest me..
Project RO - Free Ragnarok Online Server
Subspace/Continuum - Cool asteroids styled multiplayer which I'm currently hooked on.
Gunbound - Free Worms styled online game that I've played for about a year and a half.
Rose Online - Free, still in beta. It only came out a month or so ago and my friends are playing it, I need to get a new Video card..
Mouse breaker - They haven't blocked it at school, so it's all good.
That's all I can think of now. They're the main games I play when I have the time, anyway.
Notice that they're all free? Funny that..