Manchester City web site. After the club had been taken over by new owners, a decision was taken that the site had to be radically reworked, with the ambition of being the best football web site in the world. Four months later, they had their new site.
Developed using Sitecore, it's interactive, contains video, high res photos, real time updates and is integrated with many other systems such as online marketing and ticketing. It took 357 developer days using scrum cycles, an agile rapid development technique. It copes with high traffic volume in peaks, and has been translated into arabic requiring right to left reading. On match days it takes in OTPA feeds to give people live stats, live commentary, photos.
I was very impressed with what they'd achieved with a relatively small number of developers (I think there were 3 in total), and the level of integration and functionality they'd achieved using Sitecore. My only criticism was that I was trying to look at it during the talk on my iPad, and it doesn't render properly on an iPad (or on Firefox on my mac) , and the video and Matchday Centre are in flash so they don't work. We were told though that it is being rewritten into html5 and should therefore be compatible by Christmas.
As an indication of how important they see the interaction with fans, they've appointed a permanent, full time social media executive, who manages the Facebook site, Twitter, forums etc. It did amuse me slightly that they can't have live twitter feeds, or open fan forums because the language the fans use can be less than tasteful, and they get sabotage posts from Man Utd fans!