A Blaschke product is a complex function which maps the unit disk to itself. It is fairly easy to show that the finite Blaschke products are precisely the analytic functions on the unit disk D that are unimodular on the unit circle, continuous across the unit circle, and have finitely many zeros in D. For more information, see for instance my thesis, Wikipedia or one of the many books on the subject, including Garnett, Duren, and Koosis.
To help me in my own study of these Blaschke products I have developed a set of programs that visualize the behaviour of them. The programs are made available on this page as Java applets. To play around with the program, just click on its name in the menu to the left. If you have problems with the programs, first make sure that you have Java installed. If not, you can download it (you will need the Java Runtime Environment only).
Tips, comments, bug reports and other feedback is highly appreciated.