From the new Blizzard Entertainment anniversary site:

In 2011, Blizzard Entertainment marks its 20th anniversary. Over the course of the year, we'll be celebrating two decades worth of games along with you, the community of players who made it all possible through your passion, enthusiasm, and support. Join us for a stroll down memory lane on this anniversary site: revisit the history of our games; participate in our global community contest; check out the photo gallery to witness how our epic hairstyles have evolved over the years; and much, much more.

The anniversary site will be continually updated throughout the year, so be sure to check back periodically. For now there is a thank-you video from Mike Morhaime and Frank Pearce.