Minercaft is great. However if you like retro game you can try star wars knights of the old republix. (Not the online MMO one the first one) Look it it up here is a walkthrough of it. http://www.youtube.com/watch?v=63COMmvoty8
Well this isn't necessarily a downloaded game, but its super fun and free! Offensive Combat is a multiplayer online shooter powered by Unity. I would suggest having a decently strong computer for this though. PS It's got its own sence of humor have fun.