Earth Day Network
This site includes a plethora of information about Earth Day. It might be best to begin with the site map.

EarthDay.gov
The federal government's official Earth Day site; it's a portal to all things Earth Day in regards to the federal government. Learn how to take action in your home, classroom, or at work.

Events
An excellent listing of Earth Day events around the world.

Getting Down to Earth Day 2000
About.com Geology Guide Andrew Alden's "annual curmudgeonly take on this safe occasion."

U.S. EPA Earth Day
Earth Day events, history, resources and much more from the U.S. Environmental Protection Agency.