OpenTip is a javascript tooltip framework. It's free, open source and comes with different styles.
Features:
- Stems (little pointers)
- Automatic content download with AJAX
- Different styles
- Automatic repositioning of the tooltip if it's not in the viewport of the browser anymore
- All kind of triggers (The tooltip can be triggered by mouse over, click, form submit,... everything you can think of really)
- CSS3 Animations
- Well tested, with over 200 unit tests