This is a calculator to work out bike gearing in terms of gain ratios. You can feed in the crank length, wheel size, and gear sizes and it'll tell you what your gears will feel like, what your shifting ratios look like, and what level of derailer specification you should be thinking of. And, with the use of gain ratios, you should be able to compare any different random human powered vehicle and it'll still make sense.
This calculator is written almost entirely because of Sheldon "Gain Ratio" Brown and his magical gear calculator. I didn't like a few things about it, so I decided to re-implement it myself in a more modern fashion. I've got some more stuff in store, but I figured I'd toss it up to get some people actually using it. I tested it against Sheldon's calculator and it gives the same results.