Math resources on the web are many and varied. There are some comprehensive sites and some problem resources listed here, but these lists are just a starting point.
There are also many video resources available, such as
Wolfram also provides powerful tools at
List of sites with problems hosted by the Math Forum.
Math Magic: Math challenges for all levels.
Arithmetic Problems and Puzzles: - Problems and puzzles from the Math Forum.
Topology Games - The torus and Klein bottle are introduced through six simple games (a Java-capable browser is required). Simple to use (from upper elementary and up) introducing interesting and deep topic.
Sequence Puzzles - A compilation of sequence puzzle resources.