Hyperlinks to fun and challenging math puzzles, riddles and brain teasers (all levels):

http://www.greylabyrinth.com/

http://www.dse.nl/puzzle/index_us.html

http://xraysgi.ims.uconn.edu:8080/rpa-output/

http://www.math.tau.ac.il/~puzne/gif/brain.html

http://www.brainbashers.com/