Many good multiplayer games require expensive things like four controllers for a PS3 or a high-end PC. But for those of us with old computers, there are still multiplayer, non-Facebook games available.
Pixel Prospector celebrates their fourth anniversary with a video of 44 free, multiplayer indie games. There are all kinds of fighting games, Mario clones, falling block games, and an adorable Pengo remake called Push Push Penguin. If you get tired of shooting your friends, there's also a cave-jumping game called Das Uberleben.