If you try playing
Dota 2 and
League of Legends with a group of total strangers, it'll be a lot of fun. Then there's
Goat Simulator which people say is a pretty dumb game but you enjoy a lot if there's another player playing with you; you'll end up in peals of laughter. Another one which is an entire virtual world of its own is these
pet games in which you can create accounts, own virtual pets (you've even got to feed them or else they will starve), and buy virtual items for them using one of two virtual currencies. There's also
Borderlands 2 which is fun to play in a group, wherein the players are like a posse of future cowboys, traveling the land to bring the righteous hammer of justice down on thousands of bizarre outlaws.