I like like the section title recreational mathematics from MathWorld, it sounds almost like recreational drugs, as if some people get high by doing mathematics. Which I guess some people do…

Ahem, back to the link that I meant to post: animated GIFs is a collection of interactive entries in this section. It includes some well-known things like the cardioid, the cycloid, the epicycloid, and the Möbius strip, as well as my favourite geometry, the Poincaré hyperbolic disk, and also some lesser-known ones like the Steiner chain, or Langton’s ant, or the coin paradox (which I incidentally used at school this very morning).

Link via Ralf by e-mail.