Just to be clear, MonoGame has existed and been open source for a really long time now, and it's a mostly compatible successor to Microsoft's XNA; it's more of

Just to be clear, MonoGame has existed and been open source for a really long time now, and it's a mostly compatible successor to Microsoft's XNA; it's more of a platform abstraction library than a game engine, although it fills a really sweet spot between using engine and starting from scratch, IMO. There's also FNA, which is more focused on being an accurate reimplementation of the original XNA on top of exclusively OSS libraries and open APIs, although you can use it to make new games as well. Some games have actually used both to target different platforms in the past, as MonoGame is arguably buggier.