Sweet Tooltip is a jQuery & CSS3-based tooltip, not merely a one-style tooltip but seven. This imageless tooltips are crafted using CSS power, linear-gradient, box-shadow, text-shadow, :after and :before selector, etc.
Gips is a clean and simple jQuery tooltip plugin. Its simple, customizable, and easy to use. It also supports CSS themes.
qTip is an advanced tooltip plugin for the ever popular jQuery JavaScript framework. Built from the ground up to be user friendly, yet feature rich, qTip provides you with tonnes of features like rounded corners and speech bubble tips. it's completely free.
TipTip detects the edges of the browser window and will make sure the tooltip stays within the current window size. As a result the tooltip will adjust itself to be displayed above, below, to the left or to the right of the element with TipTip applied to it, depending on what is necessary to stay within the browser window. It uses zero images and is completely customizable via CSS.