I have uploaded a Really Useful Filter Design Spreadsheet to my Applications Note section : http://www.numerix-dsp.com/appsnotes/ because it no longer appears on the Music-DSP archive, where I originally found it.
It implements the equations from the excellent RBJ Audio-EQ-Cookbook and has some minor fixes.
I have used this spreadsheet for a lot of audio filter design work and also teaching students. I have tried to track down the original author, to give them the credit they deserve. If anyone knows who wrote this spreadsheet then please do ask them to get in touch.