Science

Chemical Elements
Periodic table with detailed element descriptions.
http://www.chemicalelements.com/   (review rate bookmark)

Global Warming: Early Warning Signs
Geographical map of the signs of global warming. Gives specific examples for specific areas.
http://www.climatehotmap.org/   (review rate bookmark)

Nature Publishing Group
Scientific journal, published weekly, with original research spanning many different scientific disciplines.
http://www.nature.com   (review rate bookmark)

The Exploratorium
Hands-on museum of science, art, and human perception in San Francisco. Site provides interactive online exhibits and exhibitions, activities, Webcasts, and more.
http://www.exploratorium.edu/   (review rate bookmark)