This is my first real game. It's FPS shooter with 5 weapons and same number of worlds. It was an entry to one competition for Czech and Slovak Republic called Becherovka Game 2005. Under action games it has ended on 4th place from 6. I think it is a good place, since I was only 14 when creating this game.
I didn't write the collision detection code (just some of it), lots of 3D math code was copied. But if you're dealing with only simplest collisions (like here), you probably don't need to understand that math, you can find it anywhere around web. Though in more complex projects, you should understand the math.