John Carmack's generosity in releasing his older source code resulted in a proliferation of free first person shooters in the 2000s, but Dutch programmer Wouter van Oortmerssen rejected this path to forge Cube based in the same "just for fun" spirit with which Linus Torvalds first began work on the Linux kernel.

AssaultCube, a name I bet plenty of Linux users know but haven't heard anything of in a long time. It hasn't seen a release since *checks notes* 2013. However, it's going to make a return.