- Nine math problems of varying difficulty for people who want a challenge with a separate PDF of solutions. Click to Download Problems. Click to Download Solutions.

- Cut the Knot: Puzzles, problems, and lots of other interesting math.

- Think Cool Math: Video description of puzzles and other cool math by a brilliant teacher. (video production quality is not as good as the actual content).

- Cool math newsletter: All sorts of cool math, aimed mostly at grades 8-12, but interesting for all ages.

- Wuzzit Trouble: The only good mobile math app we know of. Free, well designed, engaging iPad/iPhone game that allows you to play with math and discover patterns and ideas and challenges your problem solving skills without feeling like you are “doing math.” Addictive for kids and adults alike.

- Puzzles from Martin Gardner: Many challenging puzzles from the famous Martin Gardner.

- Art of Problem Solving: Problems, support, books, and courses for students who are interested in math competitions.

- Project Euler: A series of challenging mathematical/computer programming problems that require both mathematical insight and computer programming skills to solve.