Overlib

I’ve been using overLIB for a long time now and it’s an excellent library. Here’s a description from their site

overLIB is a JavaScript library created to enhance websites with small popup information boxes (like tooltips) to help visitors around your website. It can be used to provide the user with information about what will happen when they click on a link as well as navigational help. Not to mention that it looks cool, is stable, and has an active developer community to boot!