We can quibble about the accuracy of this tool, but it definitely an interesting way to visualize public transit data. French software developer Stefan Wehrmeyer invented a Google Maps mashup that shows how far you can travel by public transit or on foot from any given point on the map.
Go here and just drag the marker to any place, then use the time slider in the bottom right to adjust the size of the blotch.
Here's their video explaining the concept.
We originally found this tool on Capital Metro's blog.