Recently, we had the pleasure of having John Resig, the creator and lead developer for the jQuery library speak at the Boston .NET User Group meeting.
John's presentation showed the tremendous power and flexiblity of jQuery for enhancing the UI of a web page/site. I've been using it for about 8 months and never looked back. The jQuery library is one of those tools that comes along and changes the paradigm of a playing field. Combined w/ the jQueryUI collection of UI components this is a must have for web developers. Check it out!