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).