Geometer's Skethpad!

OK - I have to admit that this is one of my favorite pieces of software that I have seen for having students play with complex ideas on a computer.  For a virtual manipulative - it is far and away the best I've seen (not that I have actually seen that many; and most of the ones that I have looked at were free).  I could have played with it quite a bit longer.  It was fairly easy to use, playful, useful and insightful once I got the hang of it.  In the Cohort, our ever efficient brains eventually led many of us to strategize about what shapes to use first and what versatile ones to save for last.  I can see how this would be a very effective adjunct to teaching geometry in class; actually better than most physical manipulatives I've seen.  If my school district does not already have it when I get there (have to be optimistic), I will lobby for it!