Mindustry

Shattered Pixel Dungeon is an open-source traditional roguelike dungeon crawler with randomized levels and enemies, and hundreds of items to collect and use.

Solar2D Game Engine main repository (ex Corona SDK)

Desktop/Android/HTML5/iOS Java game development framework

Open-source Android/Desktop remake of Civ V

Open-source engine for Heroes of Might and Magic III

Terasology - open source voxel world