As a senior project, we built a first person shooter style multiplayer game. We built it with the Unreal engine. I've got to say, that engine is beautiful. Once we got the networking just right, it was actually pretty fun.

The unique twist to our game is that you essentially controlled two characters, but you could only control one at a time. Your "uncontrolled" character would simply be made invisible, but still liable to damage.

Here is a demo from very early in our process: