"William Wu":http://www.ocf.berkeley.edu/~wwu/intro/intro.html has a wonderful collection of riddles. Some are "easy":http://www.ocf.berkeley.edu/~wwu/riddles/easy.shtml#howManyLettersInCorrectAns and some are "hard":http://www.ocf.berkeley.edu/~wwu/riddles/hard.shtml#zeno
And I really like the fact that there are no answers given. You have to figure them out yourself. Or have that riddle bounce around in your head like a jingle until you go crazy.