I had the idea yesterday and wasted most of a class period putzing around getting it to work right. Sheldon's calculators are nice, but they work best with geared bikes. This is more a tool for experimenting with gearing and finding combinations that end up with similar ratios. It displays pretty much all of the possible cog and ring values I have ever seen used for FG/SS applications, but you can add or remove to suit your needs on the fly.

The main feature is that once you specify your wheel size, you also choose a target gear inchhhhh you would like to achieve, and the spreadsheet will show you which gears are higher and lower and by how much (with a variable spread) in a visual manner. Instead of reading through a chart looking for numbers close to the one you want, you can just look for the whitest boxes or know that increasingly red or increasingly blue boxes are higher or lower based on the spread.

it sounds complicated, but it's really not. Play with it and tell me what you think. if people think it's useful and should stick around, I'll neaten it up and put it somewhere more accessible.

http://grit.homelinux.net/gi/

edit:
Here are some diameters in inches for common tire sizes. I'll make a dropdown box to do this soon.
700 x 20 = 26.1
700 x 23 = 26.3
700 x 25 = 26.4
700 x 28 = 26.8
700 x 32 = 27.0
700 x 35 = 27.2

27 x 1 1/8 = 27.0
27 x 1 1/4 = 27.1

26 x 1.5 = 24.9