Awesome Math Girls
Math is Important
How to prepare?
Competition Math for Middle School
Competition Math contains over 700 examples and problems in the areas of Algebra, Counting, Probability, Number Theory, and Geometry.
Books recommended by Berkeley Math Circle (various levels)
The Berkeley Math Circle's list of required and recommended books.
Eleven Years Mathcounts National Competition Solutions
Mathcounts State Competition Preparation
Math Olympiad Contest Problems for Elementary and Middle Schools
The Art of Problem Solving Series
(Various Books for Elementary, Middle School, and High School)