NYC-Grid's Before & After photos of New York are fascinating. It's so interesting to see how vastly the city has changed, but also how recognizable it still is. I especially love the photos of spots I've been to recently or am already familiar with. It's fun to be exposed to a new perspective on something that seems so ordinary.
Many of the photos are interactive, so you can slide the yellow marker in the center back and forth to see more or less of each picture.
(Discovered via Cupcakes & Cashmere!)