Crypto challenges for puzzle fans
On the new MysteryTwister C3 web site, crypto fans can practice cracking various encryption mechanisms. Jointly created by the German universities of Siegen, Bochum and Duisburg-Essen, the site currently offers 27 challenges organised into four levels of difficulty. Level 1 tasks can reportedly be solved using pen and paper, while the higher levels require programming knowledge. The highest level, "X", contains unsolved problems and solutions would "attract the attention of the scientific community". The creators of the site are also calling for users to submit their own challenges to expand the site.
If the challenges presented on the new MysteryTwister site are not difficult or varied enough for you, consider hacker.org or projecteuler.net, sites which have existed for some time. The former contains a wide spectrum of challenges for hackers in the classical sense and the latter offers more mathematically oriented problems.