AssaultCube 

AssaultCube is a first-person shooter game for PCs (GNU/Linux, Windows and macOS) that is free software, with the caveat that some media assets are nonfree. Apart from that, the game is a lot a fun. It seems to mimick the good old days of Counter Strike when it was just a Half-Life mod, and that’s great! It’s one of the few libre, realistic FPS games out there.

At first you dabble a bit to actually hit players online. The fast pace does not make it easy for newcomers to shoot straight, but if you stay with it for a while, your skills will improve in quite a satisfactory way, which I suppose can be said about most things.

Sure, you may shake your head at characters falling from great heights and remaining with full health (not that realistic, eh?), or the lack of good graphics and animations such as you’d find in AAA games, but considering this one is gratis, runs everywhere and entertains you for hours, it’s well worth a shot.