To someone who has the time:
I think it would be very useful to develop a taxi cab fare estimator for NYC where you could plug in an origin and destination and a rough estimate of the taxi fare would be spit out. The taxi fare is formulaic anyways (flat rate + distance), with the biggest wildcard being traffic. If you could sync the estimator with traffic reports, then it would be even better. This would also help with making offers to non-yellow cabs (the so-called gypsy cabs). It could be in the same vein as hopstop.com, the great subway route resource.
If someone has already come up with this, then I apologize but I wasn't able to find anything on my google searches.