Adobe Kuler is a very pleasant and useful web application.
This application is noticeable by its design and by its integration with other Adobe products. The Adobe Swatch Exchange file format (.ase) generated may be integrated in inDesign or Illustrator for example to share the same color guideline.
A very funny feature is the Pulse community : a big picture view of color usage. Colors of downloaded themes are distributed on a color wheel. Interesting even if I don’t know the real advantage of this. Anticipate trends?.
You may also take advantage of the Kuler API (if you are a developer of course).
Before I describe more in details this application and similar web application, you can go deeper on this application by watching Adobe.TV
I rainbowly recommend this site.