This survey from Rubicon on growth of web applications is interesting. Email and games still are the most favorite/used web applications.
Also one interesting fact to notice was that people spend 58% of their time on internet of which 22% is spent on web apps, of their total time spent on the computer.
Email and Games may be the leading web apps because I feel that the majority of the crowd using web apps is pretty young and obviously email and games are on top of their list. I am sure as the crowd gets old, their needs would change and different web apps may lead the race.