The Blaschke Product Explorer is the basic Java applet I wrote to experiment with the behaviour of Blaschke products.
A Blaschke product is determined by its zeros, which are shown as black dots inside the domain (left). The mapping is visualized using colors, where the basic idea is that a point in the domain (left) corresponds to a point of similar color in the range (right). When you move the mouse pointer to a point in the domain (left), the image of the point is shown in the range (right) as a black dot.
To add a zero, click with the left mouse button in the domain. To remove a zero, click with right mouse button on that zero. To move a zero, drag the zero while holding the left mouse button.