Unciv

The automation tower defense RTS

Desktop/Android/HTML5/iOS Java game development framework

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

Open-source engine for Heroes of Might and Magic III

Solar2D Game Engine main repository (ex Corona SDK)

Scalable game server framework with clustering support and client libraries for iOS, Android, Unity and others through the C SDK.