What charting package(s) do you use? Is there an available list of the specific chart types available? Is it possible to extend the existing chart set with 3rd party charting packages?

We use ExtJS and Fusion for visualization quite a bit.  They are both great packages.  Of course, we expose mashups in many standards data formats, letting you pick any Rich UI tool.  This means you can use Flash, Flex, Silverlight, Ajax, etc.  As for our the 'out of the box' chart types, you can learn more about them in our Documentation on our Mashup Developer Community (www.jackbe.com/dev).