My favorite puzzle site is sponsored by the United States National Security Agency.  The USA Mathematical Talent Search is designed to find and encourage young mathematicians via some wonderful math problems.

Steven Finch of MathSoft's Mathematical Constants has a lot of information about all the constants you know and don't know.

Another interesting site is Mike Shenk's Puzzability.  Mike edits crosswords for the Wall Street Journal.

If you like puzzle boxes, one of the best Japanese sites is Akio Kamei's world of Karakuri trick boxes.  Even getting into the site is a puzzle.  Both English and Japanese version are available.

An excellent company for puzzles is Kadon Enterprises.  Polyiamonds, polyominoes, and many other beautiful items can be seen here.

Ken Duisenberg's Puzzle of the Week is another good place for math puzzles.

Interactive Mathematics Miscellany and Puzzles, or cut-the-knot, is a site by Alexander Bogomolny devoted to math puzzles!  A fascinating site.

The Geometry Junkyard is a fantastic site filled with hundreds of pointers to other great math sites.

For a great review of how particle physics, quarks, and leptons work, visit the Particle Data Group.

The world's oldest puzzle organization is the National Puzzler's League.  Twenty years ago, Will Shortz introduced me to it, and I've been a member ever since.

Some interesting puzzle jewelry is available at Karl Scherer's site.

A variety of nice puzzle programs is available at Jared Weinberger's site, specializing in sliding block puzzles.

Given a certain number of protons and neutrons, how stable is an atom?  This is one of the best unsolved problems in mathematics and physics.  For data, go to the website maintained by the world's most powerful supercolliders.

For people interested in the business behind crossword puzzles, visit the cruciverbalist home page.