Cool Math
Coolmath: Coolmath4kids: Scratch Addition
Demonstrates a method for easily adding long vertical columns of numbers. Includes two sample problems to try.
AAA Math
Aaa Math: Multiplication of Two and Three Digit Numbers
Explains the traditional way to multiply three-digit by two-digit numbers. Gives practice and games to practice this skill.
Wikimedia
Wikipedia: Graph Theory
Wikipedia provides an introduction and overview of graph theory. In addition to introducing the basic terms, there are related links to classic problems in graph theory such as the traveling salesman problem and Dijkstra's algorithm.
Other
Stat Soft: Statistics Glossary
Dozens of statistical terms are defined and illustrated in this glossary.