Aside from the ancient hacker.org and Project Euler my favorite challenge site. Maybe you find some joy with it, too :)

From the about-page

Inferno

For the people not familiar with challenge sites, a challenge site is mainly a site focussed on offering computer-related problems. Users can register at such a site and start solving challenges. There exist lots of different challenge types. The most common ones are the following: Cryptographic, Crackit, Steganography, Programming, Logic and Math/Science. The difficulty of these challenges vary as well. [...]

Gizmore

The goal is to make a global ranking for challengers.

Also we like to give an overview of popular challenge sites, and encourage challengers to try them out. [...]