Who has some of the coolest data around? NASA, of course. While you can dig around their numerous websites looking for gems, they have pulled together many of their best work into a free iPad App. The quality of the visualizations is incredible, and most of them are well annotated/narrated. They add 8-10 new visualizations each month.
Here are stills from some of the videos:
An addictive collection of beautiful charts, graphs, maps, and interactive data visualization toys -- on topics from around the world.