1 – Nice Titles Revised
An improved Nice Titles Tooltip Script with Accesskeys support.

A lightweight prototype based JavaScript tooltip

2 – Nice Titles
A classic. The script uses a background image.

3 – AJAX enabled Help Balloons
Help windows in baloon-design. AJAXified version is also available.

4 – jTip – A jQuery Tool Tip
Extensive AJAX-based tooltips with numerous functions and presentation [...]

The problem: loads of websites around the internet using the exact same javascript file. The file is a whopping 100KB in size. Since this same file resides on each website’s server, the file is downloaded and cached for each individual website. Lots of load time for same file.
The solution: Google AJAX Libraries APIs. Google hosts [...]