MathPickle.com is a free online resource of original mathematical puzzles, games and unsolved problems for K-12 teachers. It is supported by the American Institute of Mathematics.
"Math Pickle is a great top 10 math website for K-12 math topics. It seems to be designed with the teacher in mind, but do not let that deter you! Take mini-lessons and do them with your kids! Make it a game. Make it a fun project. They will learn without realizing it (which is the best way to learn – naturally!)"