Besides the simple CSS tooltips, Prototip is probably going to be a favorite. Looks like it’s easy to build simple and complex tooltips using the Prototype javascript framework and add effects with Scriptaculous.

FancyTooltips is a WordPress Plugin that “replaces those boring little boxes that appear next to your mouse with a dynamic and impressive one”.

Tooltip scripts with CSS and/or Ajax - something i wanted to get a list together for. If you have one to add, drop a comment.

• List of 25 Javascript/Ajax & CSS Tooltip Scripts
• 40+ Tooltips Scripts With AJAX, JavaScript & CSS: Smashing Magazine
• 24 Tooltipp-Scripts
• Simple Tooltips « Code Eazy