pwnable.kr is a non-commercial wargame site which provides various hacking challenges regarding system exploitation. There are flag files corresponding to each challenge, which must be submitted to the website to obtain the corresponding points. In order to read each flag file, you need some skills regarding programming, reverse-engineering, bug exploitation, system knowledge, or cryptography.
Introductory challenges with simple, common, straightforward mistakes.
Typical bug exploitation challenges for rookies.
Challenges which are painful to solve.
Each intended solution requires use of a very special technique.