He's shared a great slideshow he created for a 3-hour workshop introducing D3.
http://bost.ocks.org/mike/d3/workshop/
D3 is used for a huge variety of interesting visualizations.
I use D3 in all the visualization systems used in the Next Generation MW Google project.
I also use it for graphs I'm creating now for the VISUAL project and for Avalanche2D-JS.
the graphs used in the current work in VISUAL and the the .