Mapnificient is the best idea I've seen in a very long time. It allows you to pick your location, set an amount of time and then find exactly where can you go using public transportation in that time. Absolute genius.
I can see myself using Mapnificient on a daily basis, limiting my searches for bars, cafés and restaurants by time rather than space. And I can see myself using this for the next time I have to rent an apartment, to see exactly what areas are within 30 minutes of the Gawker's office, which is my maximum time when it comes to travel in NYC using the subway or the bus.
It is so damn useful that it makes me wonder why Google or Bing Maps hasn't implemented this yet. I hope Stefan Wehrmeyer makes an app for all iOS, Windows Phone 7 and Android as soon as possible. [Mapnificient—Thanks Jordi!]