I just came across this brilliant website: a list of problems (195 and counting) sorted by difficulty. They're not math problems so much as they are math tasks - there's a trillion different ways to solve each one by deriving an algorithm to suite the purpose... and best of all, their forums have user-submitted solutions for some of the tasks that are really, really ingenious.
Anyway, thought I'd share:
Project Euler
Anyway, thought I'd share:
Project Euler