Here is a very useful directory site of many Web 2.0 applications. These apps are typically free and designed for collaborating with others. The apps are organized in to categories including one for Elearning 2.0 where several very interesting learning apps are listed.
I also found the wide range of very innovation apps in the lists that I have never thought of before. There are sites to help you improve your health and network with others doing the same. Apps for networking with others is all kinds of ways.
According to lists, the most popular app is blurf a trivia question game.
What is also nice of this site is the you can read reviews and ratings on the Web 2.0 apps/sites so it can help you make sense of the new Web 2.0.