A bunch of nice folks who really like movies. They stirred up some controversy a few years ago when they compiled a list of the 100 Greatest American Films of All Time -- everyone wanted their personal favorite movie to take the top spot, which went to "Citizen Kane."
Later, they put together a list of the Top 50 Screen Legends, which inexplicably ranked Steve McQueen above John Wayne. More recently, they compiled a list of the 100 funniest movies, which inexplicably ranked "Tootsie" higher than "Duck Soup," a list of the most thrilling movies, which was pretty much on the money, and a list of the most romantic movies around.
Like I said, they seem to be nice guys, and I'm in favor of any group that's interested in getting people to think critically and intelligently about the art of film.
Lots more good info is available at their website, at http://www.afi.com.