script.aculo.us - it's about the user interface, baby!

Web 2.0 is about user interface - script.aculo.us is about Web 2.0 development.

The web page says: "It provides you witheasy-to-use, cross-browser userinterface JavaScript libraries to makeyour web sites and web applications fly. What's inside? animation framework, drag and drop, Ajax controls, DOM utilities, and unit testing. It's an add-on to thef antastic Prototype framework."

script.aculo.us is used by sites such as Apple, Feedburner, Ruby on Rails, etc.