Awesome (and useful) Mashup Example

We've all seen numerous examples of "mashups", or webmaps containing information from multiple resources, in the past year. Of course, I'm a big fan of the mashup... I'm also a big fan of finding a great mix between form and function, as anyone who's involved with cartographic design can attest to. This mashup appears to have captured this essence quite well, combining simplicity, good cartographic design and decent ergonomics. It's also open source! Oh, and did I mention that it's highly useful?

...and for you East Bay-ers