From Webmaths: Pretty cool website, a bit like Project Euler, but with more approachable difficulty level:  http://www.artofproblemsolving.com/Alcumus/Introduction.php

Sample Problem:

The leveling up system they have could be an excellent way to get students addicted to solving some novel math problems.  There are ways (I haven’t explored them) to set up an entire class of kids and monitor their progress through the levels.