TaxiWiz.com has launched a site that calculates the approximate fare for taxi rides between two addresses. It uses Google Maps to plot and calculate the driving distance between two points and uses that information to calculate the fare.
For example, here is a map of a route from Penn to Central Station in New York City:

and the calculated fare:

Could a New Yorker confirm the accuracy of the calculation?
It’s not the prettiest site in the world, but the data generated is valuable.
The service is currently available in New York City, Boston , Chicago, Las Vegas, Los Angeles, San Francisco, and Toronto.
They seem to have done their homework by taking flat fares into consideration. For example, the site explains that fares from JFK to Penn Station are a fixed $45 fare. It also calculates rush-hour fare surcharges.
A mobile version of each city’s TaxiWiz.com page is available as well.