Great resources for making your own maps with color-encoded data. Especially Wikimedia, a source for SVG maps.
How to Make a US County Thematic Map Using Free Tools | FlowingData.
UPDATED: So here’s my effort. It’s a 10-second, 20-frame movie contains 20 years of unemployment rates, from 1990-2009, for each county in the US. Unemployment rates above 20% are dark red; lesser values are the same color but with an opacity proportional to the rate * 5. The formula is min(rate*5, 1).
The frames were constructed more or less along the lines of Nathan Yau’s post linked above. Then GraphicConverter converted the frames into a movie. The main difference is that each county is colored by its own data —they are not grouped so that all counties with rates in a specific range are colored exactly the same.
The first 19 frames are annual averages; the last one is the September 2009 values. Since the national unemployment rate has been increasing throughout 2009, we can expect the annual average to be somewhat smaller for the typical county, and the dramatic change on the last frame to be somewhat less dramatic once the full year’s data are averaged in.