Cartographer: Free Javascript libraries (MIT License) for generating basic thematic maps.
 Choropleth |
 Pie charts |
 Cluster |
Links to additional live examples at the website. You can hard-code data values, or retrieve them from an online source like a Google Spreadsheet.
Cartoview: Open source software that “makes building and editing maps a breeze…”. I wouldn’t go quite that far – I think you’ll need quite a bit of experience with Javascript, Google Maps and webservers to get this up and running. But there’s a substantial amount of help available, including tutorials, online help files, and a user forum. There are also a number of online demos, with links to downloadable source code for the demos to help you figure out how to do stuff. I gotta say, some of the demos look very impressive, like this Google Earth plugin view that lets you plot points/data from online resources (Wikipedia, Yelp, Foursquare, Twitter, etc.), with user-definable interface styles:
