Brainstorm - Psychedelic raytraced graphics packed onto a display that can only show a tiny set of characters and colors. A beautiful hack.
YASIV - An interactive visualization of the connections between books on Amazon. I never found a good way to expose these sort of force-directed network graphs within a usable product, but I remain fascinated by them, they're a powerful way of communicating relationships between large numbers of objects.
Mining of Massive Datasets - Rich, detailed, and practical, this is an invaluable overview of the techniques that you can apply to big collections of unstructured data to produce useful information, and is freely available as a PDF. I'm looking forward to learning a lot from this book, I just wish I could pay them for it without ordering a hardback copy.
LoremPixel - Simple but handy service that auto-generates placeholder images for your design prototypes, with easy control over the size and category.
Map of the Drug War - Chilling and information-rich, this visualization of Mexico's violence shows how bitter the drug war has become.