Heatmaps to see flow of traffic and what people click on and where they go.
Visualizations : Survey on Income and Living Conditions in Ireland 2010
I bought the book visualize this written by Nathan Yau creator of the website Flowingdata (one of my favorite data visualization websites).
Below is one of the coolest tools I found on the program Many Eyes, I was playing around trying to program a tool like this. I knew options existed. But Many Eyes just does it so elegantly.