Perplex City (otherwise known as Project Syzygy) is an Alternate Reality Game (ARG) that launched February 2004. An ARG is a game which blends the fine line between reality and fantasy (think Michael Douglas in The Game). Most of them involve puzzle solving, with information being drip fed to players through blogs and websites (which try to maintain the illusion that the game is reality).
The storyline of this game is that Perplex City exist in another dimension to Earth. Somehow a precious item of Perplex City, known as the Cube has been stolen from the city's academy and ended up on Earth. It is up to the players to find the Cube and try to understand the mystery, with the person finding the Cube being awarded £100000.
Standard fare for an ARG. However Perplex City has several features which are quite innovative.
Most ARG are ad campaigns for large companies or smaller "homemade" projects. Perplex City is different- instead of advertisement, puzzle cards are sold through sites like firebox.com allowing the makers to be self sufficient. These cards somehow relate to the mystery of the cube and range from easy to incredible difficult to unsolvable (one of them asks you to prove the Riemann hypothesis). The cards are challenging but raise more questions then they solve.
Fortunately the cards aren't necessary to play. Like most ARGs the majority of the mystery takes place through the websites and blogs created by the puppet masters. Perplex City also has a number of live events that involve puzzle solving, team work and general stupidity (how many people can you fit into a London phone box?).
The Live Events are well orchestrated- during a scavenger hunt in London a mole amongst the players escaped by helicopter, players took part in massive text game tournament and during a celebration ceremony on the London Eye Morse code was flashed across the Thames. The general air of mystery is essential to the game and Perplex City has this in spades.
If you're interested in trying something different, I suggest giving this a good look.
References and Useful Sites
http://forums.unfiction.com/forums/viewtopic.php?t=9453- resources to understand the game quickly
http://www.tranzed.net/project_syzygy/- useful primer
http://perplexcitywiki.com/- fan made wiki