This is a little VB program I wrote that takes your stock tire size and then lists other sizes based on limits you specify.  Like maximum width over stock, minimum sidewall ratio, maximun rim diameter, and maximum % difference in height/circumference/speed variance from stock.  You can also enter a speedometer speed, select a tire size from the list and it will display your actual speed.

Here's a screen shot:
Tire Size Utility
VB 5 Source for Tire Size Utility
Tire Size Utility
Feel free to download it, copy it, share it, post it on your own web site, download the source, modify it, whatever you want.  Just don't blame me if it breaks something or you modify it to do something stupid.  Standard disclaimers, blah, blah, blah.