A few personal recommendations here: Xonotic is the most engrossing quake clone I’ve ever played. The gameplay is fabulous and it would run on a potato. Definitely check it out if you like arena shooters.
Also people who bounced off FreeCiv for its somewhat dated mechanics (I believe it’s most similar to Civ 2 in terms of gameplay), might enjoy Unciv. It’s a FOSS porr of Civilization 5 and the last time I played it a few years back it appeared to be almost feature complete in terms of parity with its inspiration.
Lastly nethack is hard as nails and I’ve never been able to finish it. Good game though!