My favorite example of this is `xneko`, which is a cute cat that chases your mouse pointer. It's a C program running under UNIX/X11 that was last updated in 19

My favorite example of this is `xneko`, which is a cute cat that chases your mouse pointer. It's a C program running under UNIX/X11 that was last updated in 1993. Last I tried, it still compiled with absolutely minimal changes (I think I had to change an #include directive or something) after typing `xmkmf; make` (which was the way to compile X11 programs at the time), and runs, on macOS. Provided you have Xquartz installed.