Macalester College Problem of the Week: Weekly math problem and archive from Professor Stan Wagon for first-year college students presented by the Math Forum.
Math Forum: Problems of the Week: Projects from the Math Forum presenting weekly interactive math challenges to accumulate points for bronze, silver and gold prizes. Individual or class participation.
Math Magic: Devoted to original mathematical recreations with puzzles and links.
Purdue University Problem of the Week: Challenging problem open to college & pre-college students, faculty, and staff. Site also contains archive of past problems and solutions.
The Little Math Puzzle: The Little Math Puzzle has been a K-12 Internet activity since 1995 for grades 5-9. Currently the puzzles have ceased but the site maintains complete archives of past problems.