Terraria is an open-ended simulation, RPG, building and multiplayer game, all at the same time! Terraria is inspired by many different games, and was released 16th of may. You can buy it on steam for 9,99$/9,99€
Terraria fully supports drop-in, drop-out multiplayer gameplay but as of right now, the sever system is like that of Minecraft, where you have to run a server program and a client program. Later they have promised that they will intergrate the Steam server system, which will make it much easier to play with your friends.
If you have anymore questions I'll kindly point you to the Home page, official FAQ, Official Wikithe unofficial wiki or the official forums.