Protovis
Since the site says it much better than I ever will, here goes:
Protovis composes custom views of data with simple marks such as bars and dots. Unlike low-level graphics libraries that quickly become tedious for visualization, Protovis defines marks through dynamic properties that encode data, allowing inheritance, scales and layouts to simplify construction.
Protovis is free and open-source, provided under the BSD License. It uses JavaScript and SVG for web-native visualizations; no plugin required (though you will need a modern web browser)! Although programming experience is helpful, Protovis is mostly declarative and designed to be learned by example.
This project is led by Mike Bostock and Jeff Heer of the Stanford Visualization Group.
An easier-to-learn competitor to Processing.js, perhaps?
Found via FlowingData.
