Penumbra 1's engine now FOSSThe engine of the best proprietary horror game in existence (Penumbra Overture) has been released under GPLv3, and some parts under even more liberal licenses. The engine features a physics interaction system, which is extremely well-balanced between complexity of possible interactions and intuitive usability. A few test maps and some objects and textures were released under By-Sa-3 (in comparison: Lugaru comes with no freely-licensed assets).
The textures that come with Penumbra
There is a guide available on how to compile Penumbra and you can get some help on #penumbra or on the official forums. The biggest task is to get rid of the legacy Newton Dynamics dependency and replace it with Bullet. This 'procedure' has been applied to DungeonHack a year ago or so.
About Lugaru: there is a thread that contains some thoughts on adding free (faif free hopefully) content to it.