Links
Online Resources:
Eric Weisstein's World of Mathematics (a.k.a.Mathworld)
The Geometry Junkyard
MacTutor History of Mathematics Archive
The Mathcad Library
MathSciNet
Textbooks in Mathematics
Wikipedia
KnotPlot
AGOLDA (Dr. Avraham Goldstein's Studying & Teaching Resources)
Allen Hatcher's free Topology Textbooks
Mathematics sites:
The American Mathematical Society
Interactive Mathematics Miscellany and Puzzles
Mathematical Sciences Research Institute
Sites affiliated with Columbia University:
Columbia University Mathematics Department
Columbia University Math Department Weekly Seminars
I.I. Rabi Research Scholars
Society of Physics Students at Columbia