Real-time physically based rendering engine for Android, iOS, and more
Rayshade raytracer
Viewer and exporter for Unreal Engine 1-4 assets
.NET wrapper for the Irrlicht Engine
Gphoton in a simple 3d modeler and ray tracer.
XNA based game engine completely written in c#
simple 3d scene viewer for windows/linux/android
sauerbraten with LuaJit2 scripting