Kingsborn

It's hard to find a free 3D First Person Shooter out there, so you can imagine the surprise when Kingsborn appeared. Its not only free but very good too. A German developer created a free FPS with decent graphics and pretty addictive gameplay (especially the Maze level).
In Kingsborn you choose the number of bots (0-10) and frags (0-100) and you can also play in Network mode with friends over the Internet.

The only downside is that instructions to the game are available only in German but you can also play the English version of Kingsborn (by running English.bat). Just in case your German isn't that good here are the main keys used in the game :

You move with the keys W,S,A,D to shoot use the left mouse button. Move the mouse to change view. Click "k" for full-person view. Press F1 for keyboard/mouse settings.