Ask Dr. Math - A question and answer service for math students and their

teachers.
Encyclopaedia of Mathematics - An online Encyclopaedia of Mathematics, by 
Michiel Hazewinkel.
MathWorld
- A comprehensive and interactive mathematics encyclopedia.
The
PRIME (Platonic Realms Mathematics Encyclopedia) -
A Math

encyclopedia
including TOPICS: Basic Math, Algebra, Analysis, Biography,

Calculus,
Comp Sci, Discrete, Economics, Geometry, Graph Thry, History,

Foundations,
Number Thry.
Math
Quotes - The Platonic Realms Interactive database of Math Quotations

from
the Math Academy.
The
Mathematical Atlas: A Gateway to Modern Mathematics -
A searchable site provides a collection of articles about the many
subfields of math and includes definitions, brief biographies, and
explanations.