HTOTM: FUSION
Original Post
Sauerbraten (Cube 2) FPS!!!!!
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/