Treemaps are a powerful data presentation technique that can simplify understanding huge volumes of information. For example, Sequoia is a disk mapping tool that can show you where your disk space is going. SmartMoney's Map of the Market shows the performance of the entire stock market. Now, The Hive Group uses treemaps to show us products from Amazon: Honeycomb/Amazon Shopping.
Honeycomb's treemaps are very slick, with user-selectable dimensions for grouping, color and size. I'm not sure if it's useful yet, but it is interesting.