I don't know very much about PS3, but it seems that multiplayer gaming is not something Sony was focusing on when they made the system--their focus was on making a powerhouse of a console. (*Awaits to be sniped by a PS3 fanatic*)

However, I DO know about XBox 360. Almost every game I've played so far on the system not only has a 1 player mode, but an online multiplayer mode and an offline multiplayer mode. The only exception I've seen so far is Call of Duty 4-- absolutely fantastic game (and seriously, I don't even like shooters, but this game is like crack to me), but no online multiplayer.