Games taken from Conway's book On Numbers and Games. The Math Forum Home Page from Swathmore is an excellent resource guide that contains a significant collection of Math Problems and Puzzles. This document is very long but very useful.
MathMol (Mathematics and Molecules) is designed to serve as an introductory starting point for those interested in the field of molecular modeling.
The Explorer collection developed by the Great Lakes Collaborative and the University of Kansas.
Mathematics Education at Nottingham has some pages for students in school, including Graphs and stories, Maths with photographs, and About the numbers in today's date.
MathMagic! A K-12 telecommunications project developed in El Paso, Texas. It provides strong motivation for students to use computer technology while increasing problem-solving strategies and communications skills.