I have seen them in person, very nice tool.
It has everything you'd ever want on it for figuring out layouts of rafters.
Here is a link:
http://www.foxmaple.com/chappellsquarepr.html

Also:
http://www.chappellsquare.com/

Last edited by Jim Rogers; 03/06/11 03:13 PM. Reason: add second link

Whatever you do, have fun doing it!