transit

Mapnificent is a mapping application that shows how far you can go from a certain point in a certain time using public transit.  For example, you can enter your workplace location and see everywhere in your city that you could live to ensure a maximum 30 minute commute to work.  Or, you could find every park within a 20-minute transit ride from your house.  

Because it depends on public transit data encoded using Google's open data standard for transit info, the Transit Feed Specification, it's only available for certain cities.  It's a follow-up to the Mapumental site, a UK version which has been in closed beta for quite a while.  You can learn more about the technology behind Mapnificent in a blog article by the creator.