qTip – a tooltip plugin for the jQuery framework. It’s cross-browser, customizable and packed full of features. (Latest update and bug fixes on March 19th, 2009)
qTip provides you with tonnes of features like rounded corners and speech bubble tips, and it’s completely free under the MIT license.
qTip comes pre-packed with 5 elegant tooltip styles for use without any extra code. You can create your own styles simply by creating a custom style in the styles object and apply it using the style.name option.
Tested with the all the major browsers on the market today.