The Bicycle Gear Calculator, by JP Martineau, is the perfect app for fixie freaks and shop mechanics.  It lets you figure gear ratios by mixing and matching front and rear tooth counts, and it also gives you the Gain Ratio, which is the distance of pedal travel versus wheel travel.
Perhaps the coolest non-essential features are the Skid Patch Predictor and Speed Analysis.  The Skid Patch will figure how many skid patches you’ll have on your tire depending on whether you’re a one-sided skidder or ambidextrous skidder (Fixies usually don’t have brakes and have to stop by jamming the pedals to a standstill, thus skidding the rear tire.  Some people can do that with both legs).
The Speed Analysis figures out your likely top speed based on a 90 rpm cadence.  This is a great tool for single speed racers, helping you calculate what gear range you’ll need for a particular XC race based on the course type and average lap times from previous races there.
You can download directly from iTunes or check out JPM’s site for more info and screenshots.  Price is $4.99.
