ASCIIQuarium, a perl-script by Robobunny with graphics mostly by Joan Stark. There are screensaver versions for OSX and Win. GIF made from this video by Mewbies.
Screenshots from Siggi Eggertsson’s new screensaver: Saver Screensson. It randomly stacks vector stencils in his characteristic geometrical mosaic style.
More Siggi on his website and Tumblr, and at text-mode.org.