Problem Sets
Interesting problem sets can be found in the following representative links:
Problems of the Week
Kid's Web - Math
MathMagic
MegaMath
Buildiing with Objects
Project List
Elliptic Geometry
Mathcad Puzzler
Math Center
Problems
Designing a 3D Scene (via Mathematica)
Discrete Math
Logic and Reasoning
For more problem sets, do a search.