Game Builder lets you collaborate with friends to create 3D multiplayer games
Source: Create 3D games with friends, no experience required
Game Builder on Steam is currently in beta for Windows and Macintosh and allows you and your friends to create 3D multiplayer games.
I played around with it, and it’s not as easy to use as the article suggests. If you have a programming class with Windows or Macs, then this tool would be nice to add to your repertoire.