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

Geniusfind - Science
http://geniusfind.com/Science.htm

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

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

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

Intute:  Science, Engineering and Technology - General Science
EXCELLENT advanced search feature!
http://www.intute.ac.uk/sciences/general/