One of my favorite programs is Geogebra. I discovered it years ago when one of my students was submitting superior graphics in a geometry class in typeset work; I asked her what software she was using and how she found it. She said “Geogebra. I just googled ‘free geometry software’.”
Geogebra keeps getting better. Still free. If you haven’t tried it, you can do so and learn more at http://geogebra.org/cms/.
At the Joint Math Meetings in Boston in January 2012, I met Marc Renault of Shippensburg University. He was presenting a poster on his work with Geogebra in which he has used the program to create a library of applets for calculus I. You can find the library at http://webspace.ship.edu/msrenault/GeoGebraCalculus/GeoGebraCalculusApplets.html; I’ve linked to several of these applets in the calculus text I’m writing, taking advantage of the .pdf format to have live links for students to follow.