The GIS geek in me just had to post about this find.
http://maps.webfoot.com/

Ducky Sherwood has combined census track information over Google Maps. While this doesn’t seem like a big deal, there are several elements of this implementation which make it unique.

Most Google Map mash-ups are point related data. For example, location of coffee shops, location of Digg postings / story submissions, location of New York city taxis. This however is data based in polygons instead of points. Allowing us to simulate GIS richness on a Google Map.

Polygons can hold value data which allows the developer to display statistical information in various color gradients. This allows for richer data visualization.

The next part of this equation is to show line data overlaid on a Google Map. The line data give the ability to show custom routes and trails. It will also allow the developer to assign line weights to various roads to distinguish road capacity.