Whats the best multiplayer game? that does'nt harm your computer and is free?

ive been bored for a while and i love multiplayer games, aka virtual worlds etc... i have a facebook to so any game would be necessary as long there no s.e.x in it XD i played: ameba pico, almost every Kidd's one like club penguin and webkinz ( that was a while ago!! o.o) XD anyways just give me some results and fast!