One of the hardest classes in grad school was a class where you make Space Invaders. You would think that would be easy, but it was actually very difficult (although rewarding in the end). From basically nothing (other than a barebones OpenGL graphics API written for C#), students were to recreate the famous game Space Invaders.
You can read more about that at the project’s page, and get the source code from there too (Windows only).