There are numerous organizations out there doing similar things, but the mysterious entity known as Cicada 3301 has garnered the most public attention for popping up on internet forums with intelligence-test scavenger hunts and offering the winners a chance to become a part of, well, whatever it is they're up to. Their stated intent was to recruit "intelligent individuals" by presenting a series of puzzles to be solved, following which you're presented another, and another, and another, to find the next.
"Hello," the original message said. "We are looking for highly intelligent individuals. To find them, we have devised a test. There is a message hidden in this image. Find it, and it will lead you on the road to finding us. We look forward to meeting the few that will make it all the way through. Good luck."
In addition to such unlikely forums as 4chan, the spooky group has planted clues for would-be codebreakers on Reddit and Twitter.
The puzzles, which focus heavily on data security, cryptography, and steganography, have become increasingly complex and arcane, making allusions to such disparate subjects as Aleister Crowley, King Arthur, insect mating habits, Carl Jung, M.C. Escher, the Kabbalah, and the Diffie-Hellman Key Exchange. The Washington Post lists Cicada 3301 in their "Top 5 eeriest, unsolved mysteries of the Internet" and the group is to this day the subject of extensive discussion, debate and speculation online. Google 'em and jump in anywhere, or go here. Theories regarding the group - and there are many - include their being connected to everything from world governmental agencies like the NSA, CIA, Mossad and MI6, to individual organizations like Blackwater/Academi, the Druids, the Rosicrucians, Silicon Valley think tanks, the University of Oxford, and Scientology.
One of Cicada 3301's most famous challenges involved a series of QR Codes placed on lampposts in many different cities of the world where they operate - among them, Dallas, TX; Annapolis, MD; Fayetteville, AK; Paris, France; Warsaw, Poland; Columbus, GA; New Orleans, LA, Okinawa, Japan, and... Miami, Florida. Glad to see the Sunshine State properly represented!