When you first launched Minetest, didn't you think it was just Minecraft? Actually not. Although their interface, style, and gameplay are almost the same, Minetest is essentially a free, open-source and cross-platform voxel game engine (written in C++ and Lua, using the Irrlicht Engine). It was created by Perttu Ahola and first released in 2010, and is now developed by a team of volunteers, with significant contributions from the community. It's available for Linux-based systems, FreeBSD, Windows, Mac, and Android platforms, and provides an API for users to create their own games and mods written in Lua.

Unity (AKA: Unity3D/Unity2D, U3D for short) is currently the most popular multi-platform free game engine developed by Unity Technologies from Denmark (mainly written in C, C++ and C#, and first released in 2005). Users with C# and JavaScript programming experience can get started more quickly.