Web 2.0 Applications

I am fascinated by the available Web 2.0 Applications. My proposal for doing a presentation on Web 2.0 Applications at the Iowa Technology and Education Connection (ITEC) conference in October was accepted so I am now looking for cool Web 2.o Applications, Gadgets and Gizmos.

What do you know about these? What are your favorites? If you have one, add it to the form on my other posting.

Here is a list from Kathy Schrock:
http://kathyschrock.net/web20/

Go2Web20 is an innovative and comprehensive index of Web 2.0 applications:
http://www.go2web20.net/