NASA's Landsat 1 has been taking pictures of Earth for most of my lifetime. Now you can explore earth by zooming in and changing the view on the more than three million images it has collected. The LansdstatLook Viewer is a great mapping tool. I like how it shows you the scale as you zoom in and out. Read more about Landstat 1 on the Earth Observatory site.
One of the coolest features is the ability to view historical data. These two images, for example, show the Athabaska Oil Sands The first was taken in 2002 when Norah and I paddled through the area. The second from 2012 shows the massive increase in productions over the last decade.
(For perspective, this picture is roughly eight miles across.)