#FreeSpace2, one of the best space combat sims of all time (if not the best), is available for free for two days at #GOG.
https://www.gog.com/game/freespace_2
The engine's code has been published as #OpenSource. It's still in active development thanks to the #FSOpen project, allowing the game to run on #Linux and #FreeBSD. There is a ton of high quality mods and custom campaigns available. Go grab it!