Banshee3D is pretty cool too - https://github.com/BearishSun/BansheeEngine. The vast majority of the work is done by a single person.
It has an impressive feature list, including: "Cross-platform support using SDL 2.0 (currently runs on Windows, Linux, Mac OS X, Android, iOS, Raspberry Piv1.3 including other generic ARM boardsv1.6, and Webv1.4 with Emscripten toolchain)"