You can design a IIR Filter with the following applet
This is an applet that allows you to design digital filter graphically. There should be three plots and one parameter panel. The three plots are: a Pole-Zero plot in z-plane, frequency domain plot, and time domain plot. Frequency domain plot includes magnitude plot, magnitude in dB, and phase plot. Use the choice widget on the left side of the plot to toggle among them. By default it designs a Butterworth approximation, using bilinear transform lowpass filter. You can change the type of filter to highpass, bandpass, bandstop by select the check box on the parameter panel.
The plots are based on ptplot .
I added some interactive objects in the plot to allow the user changing the design parameters on the plots. The interactive components are the poles/zeros (in circle and cross ) in z-plane plot, the gain and frequency of band in the magnitude plot. You can change the passband and stopband's gain and frequency by select the blue lines in the magnitude plot with your right most mouse button, and drag to your desire location. Notice the horizontal line only move along y-axis, while the vertical line move on x-axis. As you drag, there will be number displayed on the upper left hand side of the plot tell you the value. Or you can hold down on control key and click on the interactive component, a window will appear for you to enter the exact value of the parameter. Every time you change the gains/frequency, a new filter will be designed. So in the same way you can change the pole zero location on the Z-plane plot. But also you can add pole-zero or delete pole-zero by click the appropriate button.
Other functionalities are on the way, I will update this page often. Have fun!