Browsing through the App Store on my iPhone this morning I came across a fantastic little app.  Simply called the Bicycle Gear Calculator, it is a simple yet full featured application that helps bike builders and tweakers calculate gear and gain ratios.  There are a ton of other features as well.  The user interface was very well done, and very easy on the eyes.  From the developer:

Designed by a cyclist, Bicycle Gear Calculator is a complete Gear Calculator for your iPhone or iPod Touch.

Evaluate current or potential Gearing Ratios, calculate Gain Ratios, Gear Inches, and Development. Use the Skid Patch Analyzer to visualize and minimize your fixed-gear’s tire wear. Explore equivalent gearing configurations that let you reuse parts you already have.

A must have for bike mechanics, bike builders and fixed-gear/single-speed riders!

Some of the key features include:

• Gear Ratio calculator
• Gain Ratio calculator
• Gear-inch calculator
• Development (Rollout) calculator
• For fixed-gear addicts, patch analysis lets you visualize skid patches
• Gear equivalency tool lets you find equivalent or closest-match gear ratios
• Speed calculator taking into account cadence and bicycle parameters
• Supports virtually any bicycle:
  • 20T to 61T Chain Ring
  • 11T to 35T Sprocket
  • 150mm to 200mm crank arms lengths in 2.5mm increments
  • Over 40 popular tire sizes
• Simultaneous metric and imperial units*

Get more info at the developers website.