FreeGameDev

While hunting for some retro games, I came across Digger. Digger is a SDL 2.0 rewrite of Digger Reloaded, and compiles on Mac, Linux, and Windows. Prebuilt binaries for Windows are available from AppVeyor; for Mac and Linux, the build is painless using CMake. The code has been contributed by various parties, and different licenses apply to different parts, including code in the Public Domain, and code licensed under Beer-Ware, 2-clause BSD, and the GNU General Public License version 2.

I stumbled on it a while ago while looking for two player games for the arcade machine. Alas I haven't had time to do anything with it.