There's a lot of online FPS games with custom maps (I'm assuming the ET server you played on only had the default maps). Nexuiz, Counter-Strike: Source or 1.6, and so on.

Warzone 2100 is a real-time strategy game that's free. I can't speak much for commercial RTS games because I don't play them often. Medieval: Total War and Rome: Total War are supposed to be really good.

But really if you don't want to potentially play the same maps over and over or have a game that requires you dedicate a lot of time to, you've pretty much nixed most online multiplayer games beyond Yahoo games.