Some useful links:
Reference list of books on problem solving.
William
Lowell Putnam Mathematical Competition
International Mathematics
Olympiad problems
American
Mathematics Competitions Internet Home Page
Math Competitions Archive
Mathematical Problems database of over 20,000 problems.
A list of problems Professor Stanley
submitted for inclusion on the Putnam Exam when he was a member of the
three-person committee to write the 1984-86 exams (8 page PostScript or PDF file)
Some fun links:
A Klein Bottle company :)