Taxi Me is a free online taxi cab fare calculator. Just enter any two destinations and it will estimate the total cab fare between these destinations. It uses Google Map to estimate how much your cab fare will cost. All major USA and Canada cities are supported, go here to find the complete list of supported cities.
It uses the default fare settings for every city, to change these settings click Adjust Fare Settings.
It will display the result along with a clear Google Map showing the route towards your destination.
You can post the results directly to twitter with build-in twitter feature. The mobile version of this website is also available, which I believe is a great addition since most people will need to check their cab fares while on the go. Enjoy!