Cube 2: Sauerbraten
From Game Making Tools Wiki
Sauerbraten is a Quake-like first-person shooter, which includes a level editor. An interesting element is that you edit levels from the same perspective (it kind feels like Minecraft) using a combination of menus and command line.
See Also
- sauerbraten.org
- Cube 2: Sauerbraten on WIkipedia.