Cartographer.js is a simple one-line include. The library comes with a number of demonstration maps showcasing what Cartographer.js can do:
Where is Twitter?
Where is Twitter? is a Google Maps mashup of 250,000 Twitter users that uses area-scaled circles to show the number of Twitter users in different locations around the world. Clicking on any of the displayed circles reveals the number of Twitter users at that location.
This Google Maps mashup displays data from the US Census: 2008 state population estimates, rendered on a choropleth (heat map) map.