In 1972, the astronauts of Apollo 17 took a photo of earth that became known as the Blue Marble. It wasn't the first picture of the earth, but (to quote wikipedia) "released during a surge in environmental activism during the 1970s, the image was seen by many as a depiction of Earth's frailty, vulnerability, and isolation amid the expanse of space."
And that seems pretty apt today.
NASA has quite a collection of earth photography including Blue Marble: The Next Generation (Trekkies, we are everywhere), which "offers a year's worth of monthly composites at a spatial resolution of 500 meters. These monthly images reveal seasonal changes to the land surface: the green-up and dying-back of vegetation in temperate regions such as North America and Europe, dry and wet seasons in the tropics, and advancing and retreating Northern Hemisphere snow cover." (Retreating now more than ever.)
Over at the Google Earth Blog (with the lovely abbreviation of "gearth", prepare to be assimilated) some techies have taken NASA's work and turned it into an animation. (Warning: Serious processor speed needed.)
But bookmark the wicked cool Google Earth blog, people are having all kinds of fun and games (like: actual treasure hunts) using GEarth.