You have a couple hours to kill in a new city, but don't have the slightest clue how long it will take you on public transportation to get to all the good tourist destinations. Mapnificent, a Google Maps-based program that shows how far you can go using public transportation in a certain amount of time, can help.
At its most basic level, Mapnificent shows a bright area on the map that contains all the destinations you can reach in a certain time frame. The service also allows users to combine two different maps (for you and a friend) to find a bar, coffee shop, museum or other landmark that's within a common time range. You can also add a bike to an equation, or change how far you're willing to walk from a station. The service is already available for a number of major cities, both in the U.S. and abroad.
Mapnificent doesn't actually tell you how to get to the places within your transportation range, though a quick perusal of your city's public transportation maps should give you an idea. The service also can't account for uncertainties: In many cities, the amount of time a public transportation trip should theoretically take is much shorter than it actually takes. But it can, at least, give users a rough guide to their time-limited journeys. The service is also experimenting with adding time of day into the equation, which should take into account traffic as well as late-night transportation service.
The service is still useful for planning, and to a lesser extent, for bigger decisions like where to live. Because it would be a shame to move to a new place only to discover the most desirable destinations fell outside a reasonable transportation window.