Blizzcon is an annual convention hosted by Blizzard and held in the Anaheim convention center. The first Blizzcon was held in 2006. Others followed in 2007, and 2008. The convention has been larger each year, and more than 12,000 ticket sold out in less than an hour when the 2008 convention sales page went live.
See the Blizzcon 2008 page for more details about the most recent event, at which Diablo III was first shown to the public in a playable build.