I learned from a Flowing Data article about making themed maps that I can download .svg files from Wikimedia Commons and that that those are just text files you can tinker with ... like shade the counties according to data by changing the fill text! And when you're done, you can embed them into HTML like this: for browsers that support 'em!
Here's what it looks like:
Using embed code like this:
<object width="640" height="480" type="image/svg+xml" data="http://www.prototypecloud.com/linked/Map_of_New_York_County_Outlines.svg">