The Deleted City - A spatial reinterpretation of the old Geocities sites. Having data in a single large dump instead of behind an API makes it possible to do things like this with it, things that the creators could never have foreseen.
Asteroid Discovery from 1980 to 2011 - See how our knowledge about the world around us has grown with this amazing animation. At the start new asteroids appear as discrete pinpricks days or months apart, by the end they're being discovered so fast they're a solid mass, it's like a lighthouse beam hitting fog. It's not only that we're finding out more, but that the rate of discovery is accelerating.
Open data on depression treatment in London - I love seeing mass adoption of data technologies, it's this sort of democratization of the tools that makes the real difference to the world. What's special about this approach is that it's so ordinary, what used to be elite techniques are now available to people in every walk of life.
BitDeli - I haven't used this yet, but I love the idea of being able to program custom analytics code, without the hassle of having to host it myself, and with the benefit of being able to reuse other people's approaches too.
Silicon Valley poverty - Even after twelve years here, I'm still shocked by how wide the gap is between the rich and the poor in the US.