There are so many new models on the internet every day, as well as some new books in print. I have tried selections from:
The Toymaker, as well as the crazy work of Matt Hawkins. Both have had books in print, and Matt's is available on Amazon.
National Geographic had a section on origami used for astronomical engineering (Oct/09).
Also, for educators, don't miss the many geometric shape sites online (http://www.origamitessellations.com/).
Some kids like to fold something cute, and there is plenty available online, much of it Japanese. Paper Box World is a favorite - very simple!