Scirus
Gateway to scientific, technical and medical sources.
http://www.scirus.com

howstuffworks - Science
http://science.howstuffworks.com

National Science Digital Library - from the National Science Foundation
http://nsdl.org

Science.gov
http://www.science.gov

Project Noah
People from around the world submit photos of wildlife in their backyards and contribute to other wildlife projects.
http://www.projectnoah.org