Encyclopedias
Encyclopedia of Trotskyism On-Line (ETOL) - http://www.marxists.org/history/etol/index.htm
An ongoing project, aimed at providing a complete documentation about the history, the idea and the activity of the international Trotskyist movement, centering upon the figure of Leon Trotsky (1879-1940) and continuing to this day.
Wolfram MathWorld - http://mathworld.wolfram.com/
A comprehensive encyclopedia of mathematics with 13,000 detailed entries.