I installed the newest version of the Proscope HD software today, and when I opened it without the ProScope attached, but with an iSight attached, I realized that it functions perfectly with the iSight. It is pretty decent for capturing still images, movies, and time lapse movies. It also has manual image controls, and it's free!
[robg adds: I played with this a bit, and it works quite well -- I particularly liked the time lapse movie settings. As best I can tell, the software is free to use without any restrictions regarding use of a ProScope microscope.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070808114732269