Timeago: Timeago is a jQuery plugin that makes it easy to support automatically updating fuzzy timestamps (e.g. “4 minutes ago” or “about 1 day ago”).
Flexselect: Flexselect is a jQuery plugin that turns select boxes into flex-matching incremental-finding controls. Think of it as Quicksilver squished into a select box.
Facebox: Facebox is a jQuery-based, Facebook-style lightbox which can display images, divs, or entire remote pages. It’s simple to use and easy on the eyes.
jGrowl: jGrowl is a jQuery plugin that raises unobtrusive messages within the browser, similar to the way that OS X’s Growl Framework works.
Placeholder: A jQuery plugin which enables HTML5 placeholder behavior for browsers that aren’t trying hard enough yet.
Masked Input plugin
The Masked Input plugin Allows you to easily add guidelines for people to input things, and client-side validation to prevent them from typing the wrong characters. It creates a formatted layout and fills it in as they type which is really nice.
Live Query: Live Query utilizes the power of jQuery selectors by binding events or firing callbacks for matched elements auto-magically, even after the page has been loaded and the DOM updated.
Form: The jQuery Form Plugin allows you to easily and unobtrusively upgrade HTML forms to use AJAX.