If you are looking for a simple but effective gear calculator for determining the gearing on your fixie or singlespeed bicycle, take a look at the Surplace Fantastic Fixed Gear Calculator. Using the input windows in the calculator you can pick your rim/tire combination and your proposed front chain ring and rear cog.
The analysis windows gives you Ratio, Skid Patch and Gear Development and there is a section for what speed you can expect to be riding at a given cadence.
In addition, you can do your tinkering in Imperial or Metric measurements, read the results in English or French and even see equivalent gear choices for a specific ratio.
Here is a non-functional screen shot of the calculator. Nice looking to boot!
Here is another link to the Gear Calculator.