My favorite "old software that still miraculously works" is oneko, a once sort-of famous cute little cat that chases your mouse pointer.

oneko-1.1b is from 1995, and yet I was still able to build and run it natively on an M1 Pro MacBook unchanged, with XQuartz as the X server.

(I just tried again on a more recent Mac, and while it still happily builds, running it crashes the X server. That seems to be an X server bug, though.)

A lot of that old software still works. That's one of the benefits of X being so stable over the decades.

I'm sure you can still find the old SUNSITE repos archived on the 'net - tons of neat stuff to play with. I'm not sure anything requiring imake would still build, though.

It's a bit tangential, but thinking of Neko, I miss the old trend of little gimmicks like this. I remember for Windows, there were all kinds of weird widgets and toys on the desktop. I've got an old version of Windows XP Plus pack that has these weird desktop dancers that you can have dance to music awkwardly, for some reason. There were also Microsoft's own desktop agents, I assume developed for Microsoft Office for Clippit originally, but also used in XP (Rover in Search) and of course, BonziBuddy. I recall a program called Shimeji as well, though I don't know much about the origin of it; video game and anime fans would post custom Shimeji characters to places like deviantArt. And speaking of Neko, Japan really seem to take well to the desktop mascot concept. If you look around, you can find official Desktop Accessory software sold for various video game and anime franchises that were popular in the 1990s and 2000s, like Konami's Tokimeki Memorial or Broccoli's Galaxy Angel, which would do various things like add mascots to the desktop or even system dialogs, provide custom icons you could replace Windows ones with, or add things like voice clips, screen savers, or desktop backgrounds to use. But, that's almost all Windows stuff. As far as UNIX and Linux go, the X port of Neko is in fact, the only thing I actually do remember exists. (I presume the original Neko program was the root inspiration for most of the other mascot-style programs that followed.) I guess when I used Linux as a kid, I just didn't really care enough to waste time on such trivial, childish things... And on a serious note, while I kind of get why these sorts of things fell out of favor, it did add some whimsy and variety to the experience of using a computer. It would be kind of neat to see a resurgence of that sort of thing. Doesn't have to specifically be desktop mascots or pets, either; there was a gold mine of gimmicky software in the 90s and 2000s.

Likewise, the gradual decline of the screensaver has been somewhat of a bummer. On one hand, yeah, the relevance of screensavers has fallen off so much that I hardly noticed when they were gone... but again, it added some fun to computing. Hard not to miss it a little.