Good-Looking & Lightweight Tooltips: Tooltipster
Tooltipster is a jQuery plugin for quickly creating HTML5-validated and flexible tooltips.
The plugin is small-in-size (4.8kb minified), works fast and can be customized with ease.
Its look can be changed with CSS (theme support) and the position of the arrow, following the mouse, delay/period of the appearance can all be defined.
Tooltips don't use any images and any HTML can be inserted into them. Also, it works well in all browsers (including IE).
Compatibility: All Major Browsers