humane.js - A simple, modern, browser notification system.
jGrowl - jGrowl is a jQuery plugin that raises unobtrusive messages within the browser, similar to the way that OS X's Growl Framework works.
jNotify - The jQuery jNotify plug-in is an unobtrusive notification system for web-based applications. Use jNotify to inform users when specific actions have completed on your site–such as when an AJAX operation complete.
jquery-notify - A growl/ubuntu-like notification system written on top of the jQuery-UI widget factory
noty - a jQuery plugin to create notifications as an alternative the standard alert dialog
sticky - Very small and lightweight notification system, however seems to be unmaintained and abandoned now