I came across a great little app for the iPhone or iPod Touch: a bicycle gear calculator.
As you can see from the image, you can dial in your front chainring size, rear sprocket size, and crank arm length, and get the actually gearing for that combination. On a separate page you can adjust the wheel size and the tire size (the taller the tires, the higher the effective gear... by a tiny amount).
Sure, you can just go to Sheldown Brown's site and use his on-line gear calculator for free, or use the but with this iPhone app you get a couple of added bonuses. First is the skid patch calculator. This is very handy for fixed-gear riders, who may resort to skidding in order to stop quickly. If you ride a combination like 36/18, where the front is an exact multiple of the rear, you will always skid in exactly the same spot on the tire... and it will last about 1 day. (A much better combination is the 39/16 offered on the 2009 Specialized Langster, which yields 16 skid patches and a reasonable 65 gear-inches, or 5.2m development).
The final feature to mention, again for fixie riders, is a screen where you can compare what you're riding right now to another combination, perhaps based on other parts you have lying around. For example, I'm building a fixie with a 48/20 combo. I can see that if I chose the 39/16 combo mentioned above, my development would change from 5.23 to 5.31m (i.e. almost identical, but I'd increase my skid patches from 5 to 16, so my tires would last longer.
Another alternative is to use the free Rabbit, a single-speed and fixed gear online calculator, which also will do a skid-patch analysis, but the iPhone app is more portable - useful for those late-night discussions in the pub about whose gear combination is better!
The gear calculator app is $4.99 at the Apple App Store.