Electronic Journal of Combinatorics (if that link doesn't work, try this instead). The journal keeps a list of home-pages of many people who study combinatorics.
For information on copyright policies of various mathematical publishers, see Greg Kuperberg's copyright page or William Stein's copyright page.
The University of California at Davis math department (this beautifully designed web-site is, or should be, the wave of the future).
All the other math departments in the United States.
Over a thousand math departments in over a hundred countries.
The Mathematical Sciences Research Institute
The Center for DIscrete Mathematics And Computer Science (DIMACS)
The American Mathematical Society
The Mathematical Association of America
The Geometry Center
Workshop on Organic Mathematics.
The Front for the xxx Math Archives
Jeff Miller maintains a website titled "Earliest Known Uses of Some of the Words of Mathematics".
Find out your mathematical genealogy (or anyone else's).
Neil Sloane's On-Line Encyclopedia of Integer Sequences is the web-version of his sequences server (which is in turn the on-line version of his Handbook of Integer Sequences).
A source of on-line books on mathematics.
A directory of on-line books on mathematics. See also the GotMath.Com directory of on-line books on math.
A review of the movie "Good Will Hunting".
A web-page about math in the movies.
Math typesetting for the Internet (see also the TeXpider web page)
Penrose tile program
Math-Net Links to the Mathematical World
David Griffeath's Primordial Soup Kitchen
Lifepage (main page for current research on Conway's game of Life)
The Bingo Hideout Games Research page on Famous Unsolved Mathematical Problems
The sci.math.research archive (with lots of useful pointers to other places).
George W. Hart's Pavilion of Polyhedreality (and his associated Virtual Reality Polyhedra page.
David Eppstein's Recreational Math page and Geometry Junkyard.
Tom Ward's ergodic theory page.
Roy Lisker's link to the Grothendieck biography project, and the start of his own translation of Grothendieck's mammoth autobiography "Recoltes et Semailles". See also Max Lipyansky's Grothendieck Circle web-site.