I am currently trying to find a good series of maths tasks that I can give to my upper maths students to really challenge them at home and am not really having a great deal of luck.  I did however come across a great site here that has some extension activities correlated to everyday maths curriculum. 

Hold the Phone I just found 2 more good ones

mathschallenge.net

University of Washington Challenge of the week

And here are 2 more additions

http://mathematicsstarters.wordpress.com/challenges/

http://mathbits.com/caching/mathcachedirectionsopen.html

I would love to hear of any that you know of and will definitely share them here.