Mapnificent [mapnificent.net] by Stefan Wehrmeyer reveals the specific geographical area one is able to reach by using public transport from any point in a city, and within a given time. The service is currently available for most major cities in the U.S., and for quite a few other cities worldwide. One simply chooses a specific location, or places a draggable pin on the online map, selects a specific time span, and the reachable zone is highlighted on the map.
Interestingly, these time- and location-based queries can also be combined with a 'traditional' search for local services, like 'coffee places' or 'musea'. Accordingly, one can figure out a good restaurant that is located within 15 minutes travel time of a public transport bus, for instance.
For those interested, the service is conceptually closely similar to Mapumental.