I am sure that most of you already know the "Prototype Windows" project that provides a very powerful way to create windows and dialogs inside your Web pages. If you do not know it, take a look it's awesome!
Sebastien Gruhier, aka Mr Proto, and others have done a terrific job allowing developers to integrate exciting features with few lines of code. Find more on the PWC site or on Mr Proto's blog.
In addition to the Windows API, you can also take a look to the Tranparent Message, Prototype Carousel and Prototype Graphics projects