The Great American Beer Festival has been held annually in Denver since 1982. The first iteration of the festival attracted 800 guests who were welcome to try 40 beers from 22 breweries. Things have changed. The event has moved to the Colorado Convention Center in Denver and over 600 breweries now pour 3,100 different beers over the three-day event. The demand amongst brewers to participate is so great that the Brewer's Association now has to use a lottery system to decide who gets to pour. It's not any easier to get in for festival-goers. Despite its ability to hold a staggering 49,000 attendees the festival sells out in minutes.
In most cases a crush of humanity like this would preclude us from even thinking of attending, but sometimes more actually is better. Brewery booths are clustered with other breweries in their region so the layout is ideal for festival-goers to sample beers from far flung areas of the country that they're not typically exposed to. There are also ample opportunities to attend book signings and educational seminars concerning all things beer.
More info: The Great American Beer Festival takes place each fall in Denver Colorado, the 2014 festival is slated for October 2nd through 4th, greatamericanbeerfestival.com.