Reference » Encyclopedias
Symbols.com - http://www.symbols.com/
Contains more than 1,600 articles about 2,500 Western signs, arranged into 54 groups according to their graphic characteristics.
Wolfram MathWorld - http://mathworld.wolfram.com/
The web's most extensive mathematical resource, provided as a free service to the world's mathematics and internet communities as part of a commitment to education and educational outreach.