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 siteāsuch as when an AJAX operation complete.
smoke.js - A framework-agnostic styled alert system for javascript.