Discrete Math & Graph Theory

Dictionary of Algorithms, Data Structures, and Problems
http://www.nist.gov/dads/terms.html
Modular Arithmetic
http://www.cut-the-knot.com/blue/Modulos.html
The Traveling Salesman Problem
http://www.pcug.org.au/~dakin/tsp.htm
TravSalesProbBIB
http://www.ing.unlp.edu.ar/cetad/mos/TSPBIB_home.html
The Hamiltonian Page
http://www.ing.unlp.edu.ar/cetad/mos/Hamilton.html