I've been monkeying around with GeoGebra for a couple of days. It is like Sketchpad, but free and open-source. So far I like GSP better. On the other hand, GeoGebra is free and my students could have it at home... And of course if I can get the applet to work in my previous post, I might have a way to post GeoGebra files on webpages as applets, something that GSP doesn't really offer. We'll see how this works out.