Sony's MAG, an online team shooter currently in beta, promised right out the gates that it would support 256 players. Sadly, while in beta, the game hasn't had anywhere near that many players. Will soon, though!
By the time you're reading this, the game's beta will have been upgraded to v1.46, which along with some balancing and a reduction in the size of the game's initial install, also finally unlocked the game's "full" multiplayer mode. Namely, the one that supports 256 players at once.
Those currently in the beta can try it out right now. If you are, and you do, drop us a line, let us know how it goes. Needless to say, we're curious.