I also wrote a Minecraft Classic server back in the day, in 6 hours as a challenge:
https://git.sr.ht/~sircmpwn/classic6
Fun times 
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I <3 these two! Haven’t had much time to play them recently, alas.
That reminds me, there’s also this
Oolite
A GPL-2.0-or-later clone of the Elite space-trading video game (ahh, remember the 1980s?). It’s pretty much the only GNUstep app I’ve actually used!
I should try and get this packaged in Fedora again (and have an excuse to play this). The last time I was stalled because the packaging guidelines at the time are really strict about bundling - and Oolite bundles some JS modules (had the same issue packaging the Budgie desktop, gah). Luckily things are more reasonable now.