Up until recently, WoW was supported on the G5. Blizzard thinks that all PowerPC Macs struggle to run WoW. I believe there's still Everquest though. As far as I know, that's about it. You can get Diablo II and play that online though. That's about the best "mmo" that you'll get these days on PowerPC. There's also Warcraft III. You can install that and get the DoTA mod.