eRuv: A Street History in Semacode - This "digital graffiti" project is installed along the route of the former Third Avenue elevated train line in lower Manhattan. The train line was dismantled in 1955 but using Google Maps you can re-explore it. Using semacodes, the former boundary is reconstructed and mapped back onto the space of the city. Pedestrians with camera phones can then access location-specific historical content linked through the semacodes. This is a very cool idea and a great use of Google Maps to explore urban history!
The Worst Places in the World - This site has been up for a few months and features satellite images, balanced, researched commentary, and photography that show the indisputable kinds of human impact that you can see from space. It's called "The Worst Places in the World".