Wikipedia can be a useful starting place for topics in math. The quality varies, so consult references to validate. References may also lead to additional valuable information.
A free resource on math topics by Wolfram Research, makers of Mathematica. More akin to a math dictionary than an encyclopedia. References and direct links to Wolfram Alpha and animations.
The Princeton Companion to Mathematics by Timothy Gowers (Editor)
The Princeton Companion to Mathematics surveys the most active and exciting branches of pure mathematics. Accessible in style, this is an indispensable resource for undergraduate and graduate students in mathematics.
CRC Concise Encyclopedia of Mathematics by Eric W. Weisstein
The CRC Concise Encyclopedia of Mathematics is a compendium of mathematical definitions, formulas, figures, tabulations, and references. (NOTE: In print in the science library reference section).
The Princeton Companion to Applied Mathematics by Nicholas J. Higham (Editor)
The Princeton Companion to Mathematics by 
The Princeton Companion to Applied Mathematics by 
Encyclopedia of Applied and Computational Mathematics by 
