Extensive list, based at the University of Florida. Includes academic sites that had particularly good indexes.
http://www.math.ufl.edu/math/math-web.html
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MathNerds: Free help with your math question in about 24 hours. Ask a question and get an answer, or just browse this
online resource for students, parents, teachers, office workers, researchers,
and anybody who needs help with or is interested in mathematics. http://www.mathnerds.com/mathnerds/links/links.aspx