Cube 2: Sauerbraten

Cube 2: Sauerbraten is a pretty cool game.

If you’re familiar with forge mode from Halo 3 and minecraft, it has an edit mode with elements similar to both, but older and not quite as user friendly. It’s more powerful though, since you have total control over the environment, and you can do simple editing with cube subdivision, allowing you to split larger cubes into smaller cubes and edit the subcubes. You can also do true inclines and such. (You aren’t limited to cubes)