The man page is dated 1993, but it also talks about VAX/VMS programs by the same name, so I’m guessing
rain is a good deal older than that.
And that’s about all it does, although that hardly does it justice. The animation effect is something you have to see yourself.
rain takes only one flag —
-d, for a delay in the animation — and personally I would recommend around 135 for a decent show. The man page says 120, which is probably fine.
And where might you find this marvel of modern technology? In bsd-games, of course.
And what can you do with this marvel of modern technology? Use it for a screensaver, of course. ;)