Puzzles and mathematical recreations
Like most Science students, I find fascinating the ingenuity that lies beneath the, in many cases deceptively simple, puzzles and mathematical recreations. Here I have collected some with pointers to many more.
Soviet Union mathematical competitions: The problems for the final part of the USSR mathematical competitions from 1961 to 1986. They are aimed at 12-14 year-old students, so they do not contain advanced math; nevertheless, they will certainly give pause to more advanced readers.
M. C. Escher: The mathematical artist par excellence, even if not on purpose.
Interactive mathematics miscellany and puzzles: A. Bogolmony mantains this site which contains many nice (and interactive!) mathematical puzzles.
Thinkfun: An on-line shop with many kinds of puzzles and brainsteasers, and some on-line games.
Crimson room: Nothing to do with mathematics, but an interesting puzzle anyway. Beware...it may become frustrating.