A collection of useful jQuery plugins I've been using or recently discovered.
1. jQuery UI
http://jqueryui.com/
The official UI library with draggable, resizable, sortable, selectable, accordion, datepicker, dialog, progressbar, slider, and tabs. Essential for any project.
2. Cycle
http://jquery.malsup.com/cycle/
The most versatile slideshow plugin. Supports dozens of transition effects (fade, slide, scrollUp, zoom, flip, etc.), auto-play, pause on hover, and responsive layout.
3. Fancybox
http://fancybox.net/
An elegant lightbox/modal dialog plugin. Handles images, HTML, iframes, and swf files. Smooth animations and keyboard navigation.
4. jQuery Validate
http://jqueryvalidation.org/
Form validation plugin from Jörn Zaefferer. Built-in rules for required, email, URL, number, min/max, equalTo. Easy to extend with custom rules.
5. Color Animations
http://plugins.jquery.com/color/
Extends jQuery's animate() to handle CSS color properties (backgroundColor, color, borderColor, etc.) with smooth transitions.
6. HoverIntent
http://cherne.net/brian/resources/jquery.hoverIntent.html
A smarter hover event that waits for the cursor to slow down before firing — perfect for dropdown menus where you don't want accidental triggers.
7. lazyload
http://www.appelsiini.net/projects/lazyload
Defers loading of images that are outside the viewport. Significantly improves page load time for image-heavy pages.