Combinatorica is a library of 230 functions turning Mathematica into a powerful tool for graph theory and combinatorics.
http://www.combinatorica.com/
Interactive Mathematica notebooks developed for the two senior high school years, associated with the book Experiments in Mathematics Using Mathematica (Springer, 1995) by C.T.J. Dodson and E.A. Gonzalez.
http://www.ma.umist.ac.uk/kd/ednet/maths/mathematica/
Various Mathematica-related demonstrations.
http://www.mathconsult.ch/showroom/showroom-E.html
Technical notes about built-in symbols
http://www.wolfram.com/support/Kernel/Symbols/System/Sum.html
Supplementary help browser.
http://www.verbeia.com/mathematica/tips/Tricks.html
Mathematica 3.0
http://www.astro.queensu.ca/~lake/mm3/mm3.html
A collection of Mathematica v.2 routines by Peter Olver.
http://www.math.umn.edu/~olver/omath.html
Java-based Mathematica interface to any JDBC compliant database server. GPL.
http://www.petroff.ch/mconnect/