modalBox.js is a very light weight modal window plugin packed with only most used features and options. Its overall size is around 5.3 kb ( 2.5 kb minified). You can easily extend it according to your need.
PicTip is a jQuery plugin that adds handy tooltips to your images, slideshows, sliders and more.
ChatJS is a full-featured, lightweight, Facebook style jQuery plugin for instant messaging. ChatJS also comes with a server side implementation for ASP.NET/SignalR out of the box.
Supports: Firefox, Chrome and IE9+ (works in an ugly way which the users deserve)
Portfoliojs is a lightweight jQuery gallery plugin for your beautiful images with horizontal scrolling which supports desktop, tablet and mobile browsers.
ResponsiveSlides.js is a tiny jQuery plugin that creates a responsive slider using list items inside
<ul>. It works with wide range of browsers including all IE versions from IE6 and up.
Biggest difference to other responsive slider plugins is the file size (1kb minified and gzipped) + that this one doesn't try to do everything.
Also Picbox WordPress plugin is now available.