This is my new favorite site. It has 5 problems of the week that are designed for algebra, middle school, elementary and geometry.
Students receive feedback and their names are posted on the site if they are correct.
There are also archived problems from the entire history of the site. Students, as well as teachers, can try ther hands at logic puzzles, number and equation puzzles, weight puzzles, mazes and much more. This is Texas Instruments' educators site. It has many teacher resources, information on new technology, lesson plans and links to helpful internet sites for additional resources. It has many teacher resources.