Minecraft? We could have our very own Zelaron server.

Alternatively, Notch has started working on a SF game, which will probably follow a similar business model to Minecraft. He's said that it will feature multiplayer: www.0x10c.com

For the general populace, however, something like Guild Wars 2 is a decent idea (not out yet, but it went up for pre-order last night), particularly is it's likely to be a big game, and it's free to play.