Cicada 3301 s a really good one. Essentially it's a secret society of cryptographers that are constantly trying to recruit members by administering complex cryptographic puzzles via clues around the world.
Cicada 3301 is a name given to an enigmatic organization that on three occasions has posted a set of complex puzzles to recruit "highly intelligent individuals" from the public. The first Internet puzzle started on January 4, 2012, and ran for approximately one month. A second round began one year later on January 4, 2013, and a third round is ongoing following confirmation of a fresh clue posted on Twitter on January 4, 2014. The stated intent was to recruit "intelligent individuals" by presenting a series of puzzles which were to be solved, each in order, to find the next. No new puzzles were published on January 4, 2015. The puzzles focused heavily on data security, cryptography, and steganography
The ultimate outcome of all three rounds of Cicada 3301 recruiting is still a mystery. The final known puzzles became both highly complex and individualized as the game unfolded. Anonymous individuals have claimed to have "won," but verification from the organization was never made and the individuals making the claim have not been forthcoming with information