This is "Mapnificent"

The idea being that you can figure out, for example, which neighborhoods would be within a 30-minute commute of your workplace.

Mapnificent is a net tool that shows the area that can be reached by walking or public transit from a given point.

The video below explains how it works. Thanks to Andrew Sullivan.

And more, if you liked Mapnificient, I’m sure you'll love http://old.isokron.com

It is only operating for Paris and another French city, but the color coding is nice and the feature finding the best points for two people to meet is very useful.