Elysian Shadows

I personally think it was a pretty good game of course it could have been A LOT better but the source code and the sdk is what makes it really fun for me.

Other reasons:

1. Who doesnt love killing aliens?

2. Reason number 2.

3. It's FREEEEEE!!!!

4. I'm addicted to any game with a map editor.

ammoman you should buy starcraft 2 it has a very extensive level editor thing. And my friend said he loved alien swarm