ArmorPaint is a stand-alone app for physically-based texture painting that aims to become an open source alternative for Substance Painter. As the name already suggests, it's brought to you by the people behind the Armory game engine.
While you can get a pre-built copy from Gumroad and support the project that way, you can also build your own version. In this video, Jayanam shows how to achieve this on Windows with Visual Studio 2017. Instructions for other operating systems are welcome!