Mapnificent shows you the area you can reach with public transport from any point in a given time. It is available for major cities in the US and world wide.
You may be interested to watch a video about what Mapnificent can do, read a blog post about how Mapnificent works or jump to the Mapnificent API Documentation.
Mapnificent was originally inspired by MySociety's Mapumental which is sadly still in private beta.
Mapnificent was created by StefanWehrmeyer.