I am a complete and utter nerd, so when I saw the pattern - by the Institute for Figuring http://theiff.org/press/interweave.html - for a crochet model of the hyperbolic plane I just had to make one. Turns out they're really good practice for Increasing as well as looking cool.
They're easy, and once you've made one you can alter the pattern to make even stranger looking shapes, or join the Hyperbolic Coral project.
By the way, here are some photos of my hyperbolic piece so far to get you interested -
this is after about ten rows with a C hook and really cheap, coarse yarn, increasing every 4th stitch