Eric Weisstein's Treasure Troves of Science
Online encyclopedias of astronomy, math, physics, scientific biographies, and scientific books, with many equations and formulas.
http://www.treasure-troves.com/
STN International on the Web
An online scientific and technical information service (subscription required) that provides access to more than 200 in-depth databases in science and technology to offer direct links to the scientific literature, patents, and chemical catalogs.
http://www.stnweb.org
