. jQuery tools tooltip
Easily create simple or complex tooltips, adding some cool effects to it.
http://flowplayer.org/tools/tooltip/index.html

HTML Tooltip
Inline HTML Tooltip lets you define rich HTML tooltips that are embedded directly inside your webpage using css.
http://www.javascriptkit.com/script/

Cool Tooltips
Lightweight unobtrusive JavaScript web-browser tooltips replacement technique.
http://okonet.ru/

Mootooltips
Display tooltip messages from HTML elements, Ajax calls or text added manually.
http://www.php-help.ro/examples/mootooltips-javascript-tooltips/

BlogFish’s tooltip
Totally flexible tooltip with Prototype.
http://blog.innerewut.de