The Global Game Jam (GGJ) is the world's largest game jam event occurring annually in late January. GGJ brings together thousands of game enthusiasts participating through many local jams around the world. GGJ is a project of the International Game Developers Association (IGDA). GGJ 2012 is scheduled for January 27-29, 2012. Registration is still open, so if you want to participate, REGISTER NOW.
If you're in New York City, NYU's Game Center is hosting a jam on-site! Space is limited, so Sign up here asap: http://globalgamejam.org/sites/2012/nyu-game-center, and be sure to follow all the directions and register at the Local Registration Site.