Remote Mouse - Turn your smartphone into a wireless touchpad and keyboard now!
Grit Game Engine - The Grit Game Engine is an MIT-Licensed Open Source Game Engine that is highly scriptable via Lua.
KDE Connect - Integrate Android with the KDE Desktop
Lumix Engine - Open source 3D game designer and engine written in C++.
Monect - Make the best use of sensors on smart device to control PC, simulate steering wheel, game-pad...
MOS - Lightweight game engine.