Flying Pig Website
They (mainly he) make and sell cardboard kits that move in interesting ways.
Obviously, most of them are for sale, but he has some free models for downloading, which change very occasionally.
Probably the most interesting parts of Flying Pig for this site, though, are his Workshop Notes blog, where Rob updates readers on the process he goes through developing his models (he's part way through a dinosaur at the moment, and has released a kids range).
He also has a section of mechanisms, with animations of all the paper mechanisms he uses, plus several that don't go into paper models (like a stirling engine).
Generally, it's a great resource for anybody interested in engineering, paper or otherwise.