Here are some interesting and useful math-related links.
Eric Weisstein's World of Mathematics
A great general resource.
The Prime Pages
Prime number research, records, and resources, maintained by Chris Caldwell, at the University of Tennessee.
Fibonacci Numbers and the Golden Section
Ron Knott's Surrey University multimedia web site.
The Mathematical Atlas
Includes a clickable index map of mathematics, based on the Mathematics Subject Classification (MSC).
The Online Encyclopedia of Integer Sequences, sponsored by AT&T.
The Mathematics Genealogy Project
at North Dakota State University.
The MacTutor History of Mathematics archive
Contains indexes on biographies, famous curves, and history topics.
American Math Society
Math Association of America