Arcanum http://www.arcanum.co.nz
*gone (http://lightning.prohosting.com/~thegame/)
~fongdev/challenges, some probably out of date
http://hammer.prohosting.com/~fongdev/challenges/
mathschallenge.net Project Euler
http://mathschallenge.net/index.php?section=project
http://www.ngsec.com/
http://www.net-force.nl
http://www.osix.net/modules/geek/
http://www.pythonchallenge.com