http://Dcpu1.cs.york.ac.uk:6666/fisher/mkfilter/
This is an interactive filter design package, for designing digital filters by the bilinear transform method. Users can fill in the form and press the 'Submit' button, and a filter will be designed for them. The source code (in C) for the programs that do the work is available for browsing or down-loading. The graphs are produced with the help of the excellent gd GIF manipulation library from Quest Protein Database Center.