The OpenHeatMap site lets you create heat maps and choropleths from uploaded spreadsheet data (CSV format), or Google Docs Spreadsheet data (which makes it continuously updatable). It supports location coding by latitude/longitude coordinates, or by a large number of name/code attributes (e.g. address, FIPS code, zip code, state, province, country). And as a bonus, you can also have time as a variable, letting you create animated heatmaps or choropleths. Here’s a snapshot of a time-animation of US unemployment by county (see the live map here):
There’s a gallery of sample maps, some of which are embeddable, like the map below of fast-food locations in the US. It’s still a little buggy; the map on the gallery page lets you select between McDonalds, Subway, and Chik-Fil-A, while this embedded map doesn’t show those options.
Here’s a short video from Pete Warden, developer of the site, with a intro to the service: