Ok, so there's this open source game called Sauerbraten. It's basically along the lines of something like the Quake series, but it's completely free! It's mainly a multiplayer game, but there are some single-player challenges. Did I forget to mention that there's an in-game level editor? Well there is. It's pretty sweet, I play it all the time. Along with level creation, it has 50+ levels! Huge variety.

Screenies:




Main page: http://sauerbraten.org/