Robocup is a good starter subject I guess. Me and four friends competed last year in the World Championship RoboCup, 3D Simulation League (under the name LittleGreenBats). It was really exciting. It basically is a virtual soccer match in which our team had to compete against a whole lot of other teams from all over the world. We ended about 20th. Which really isn't all that bad. Most of all it was just fun! This year we're trying again. We've been at it since September. We redesigned most of our architecture and it is starting to come together.
The only pitfall is that it is decided (by the league) that this year we need to play with humanoid players (last year it was more or less floating spheres), which means that we need to program them to run and to have any chance we need them to run really fast and precise too.. We need to qualify no later than the 10th of March, which is pretty quick and as of yet only one team member has succeeded in running the server that supports the humanoid players... But still I think we are going to make it!
I'll post any progress here! So come back if you're interested..