D3.js is a small, free JavaScript library for manipulating HTML documents based on data. D3 can help you quickly visualize your data as HTML or SVG, handle interactivity, and incorporate rich animations into your pages. You can use D3 as a visualization framework, or you can just use it to build dynamic pages. Unlike other systems, D3 doesn’t tie you to a proprietary representation, so you are free to exercise the full capabilities of modern browsers.
See http://mbostock.github.com/d3/ to learn more!