Calculate taxi fares on any route (limited cities)
I came across this taxi fare app today, World Taximeter which will calculate the fare on any route when you put in your start and end point. The app uses the published taxi rates along with distance, estimated travel time, minimum fare, airport surcharges, environmental charge and anything else relevant. On the few tests I put it through on routes I knew it seemed within a reasonable margin of error all things considered.
Unfortunately only London is available in the UK but really this app is going to be more useful when traveling to a foreign country (which London can be!). There are currently only 16 cities supported but they are all major ones - if you are traveling to New York for example this could be great info to have.