How 'bout a little Earth photography for Earth Day. Sure, we've been sucking up the Google Earth images of various cities and neighborhoods for a while now, but there's nothing that beats the literally stellar database of photos NASA has compiled in its five decades or so of exploring outside the limits of our atmosphere.
From satellite imagery to snapshots taken by astronauts from the surface of the moon, they have more cool Earth photos than you'll have time to pick through. But, if that's the case, you can just log onto their Image of the Day page and they'll pick a new photo for you every day from a library that runs the gamut from beautiful shots of Earth's topography to images of man's effects on the planet we call home.
So dig in a bit, but be warned: You will get nothing done once you do. The NASA image library is a timesuck of the highest order.