Reference and Encyclopaedic
Links to websites that provide background and information on different mathematical topics, including cryptography, shape, Fibonacci and prime numbers.
Geometry from the Land of the Incas is a treasure trove of articles relating to all things geometry.
The Interactive Mathematics Miscellany and Puzzles (Cut-The-Knot) contains a large collection of well written resource pages.
Dr. Ron Knott's website dedicated to Fibonacci numbers, including resources and links to other related websites.
The Prime Pages contain more than you would ever need to know about prime numbers.
Eric Weisstein's Concise Encyclopaedia of Mathematics is a valuable reference tool, hosted by Wolfram Research.
Harvey Mudd College's Math Fun Facts.
MathsNet UK offers a comprehensive A-Z of links to other mathematical webistes.
Math Forum: Library
Math Forum: Ask Dr. Math
Wolrd!Of Numbers features an amalgamation of randomly gathered numbers, curios, puzzles, palindromes, primes and other gems.
Terry Ritter provides an excellent learning guide to cryptography.
Tom Gettys presents a useful collection of pages dedicated to polyhedra, including the Platonic and the Archimedian solids.