Stela Anguelova
Youngsters seem to enjoy a fun challenge. It is our job to guide them to the right resources and to keep the experience pleasurable.
Sites
For Younger Learners
Books Recommended for Middle School Students
Books Recommended for Elementary School Students
For Older Learners
Cornell University Resource Page
MathLinks (covers middle school and high school topics and contains a multitude of resources - from forums to challenging problems even from international competitions)
USA Mathematical Talent Search (contains interesting geometric proofs & past problems)
AMC (American Mathematics Competitions)IMO (International Mathematical Olympiads)
William Lowell Putnam Comptetitions
HiMCM (High School Modeling Contest)
Interdisciplinary Contest in Modeling (ICM)
Books Recommended for High School Students
Other Fun Sites
Visit these sites if you want to find nice activities that can complement the mathematics curriculum or you are simply looking for some fun
http://cut-the-knot.org/index.shtml
Sites