Asciiquarium
An aquarium in your terminal, drawn entirely in ASCII — fish, sharks, ducks and a castle on the seabed.
Kirk Baucom wrote it in Perl in the early 2000s, on top of his own Term::Animation module. Fish of various sizes drift across the screen, bubbles rise, a shark turns up and eats whatever it can reach, and every so often a ship or a whale passes along the surface. It is all plain characters in a terminal, in colour if the terminal can manage it.
Its usefulness is roughly nil, which is rather the point. Asciiquarium belongs with cowsay and CMatrix: small tools sitting in package repositories to prove that ASCII art is alive and well, and to give an idle server console something to do.