Open 3D Engine
From Game Making Tools Wiki
Lumberyard is a free, open-source (but with a restrictive licence) game making tool recently released by Amazon (yeah, that Amazon). It's pretty Unity-like, and based on CryEngine.
Documentation's already in a fairly useful state!
Links
- Wikipedia:Amazon Lumberyard
- dungeonscript.farbs.org
- Tutorials on the official forum.