A nice roll-up page listing many useful JS libraries, including most but not all of the major players below can be found here
There are general purpose libraries. Here are some links to libraries that are more specialized:
Busy.js – a library that allows you to easily create Ajax style “Loading” animations without requiring an animated GIF or other resource. Very flexible.
http://digitarald.de/projects/ – This guys has some truly wonderful JS controls and libraries, such as a Popup Notification mechanism called Roar (based off of MacOS Growl) that pops up nice, clean Outlook style notifications on your web application. The JS library this guy uses is mootools.
Separation of Concerns using JQuery – a quick overview of removing some of the awkward duplication in a ASP.NET repeater using a JQuery. The solution presented certainly doesn’t cut any lines of code out of your solution, but may be a better factoring of form and function.
Partial Rendering with ASP.NET MVC and jQuery – this is great example demonstrating a few things, but particularly rendering a user control as a view in MVC and having JQuery append the results to a list. The scenario is a task list where an ajax call adds a new task to the model and the resulting task data is sent back to the list to be appended, rather than reloading the whole list.