Puzzle Hunt
Second only to Capture the Flag with Stuff and the Underground tour Puzzle Hunt tends to get the most non-KGB people involved with it.
It often takes Way Too Long but is also Lots Of Fun
Past themes:
Spring 2007 - Soviet Alice in Wonderland
Fall 2007 - Where in the World is Carmen San Diego
Spring 2008 - Hackorp / Trust Busters
Fall 2008 - Psychic Agents for Geller Hotel
Spring 2009 - Time Travel
Fall 2009 - Clones
Spring 2010 - Adventuring Party in a Wizards Tower
Fall 2010 - Hunt didn't happen this semester because we were all lamers
Spring 2011 - Film Noirey Detective Story
Fall 2011 - Aperture Science: Doomsday edition
Spring 2012 - Myst-Inspired "The Great Library"
Fall 2012 - The Weathermen
Spring 2013 - Pirate Treasure Hunt
For a great example: A Maze of Twisty Passages (KGB Puzzle Hunt)