Tooltipster is a lightweight (4,813 bytes minified) jQuery plugin that enables you to easily create clean, HTML5 validated tooltips.

Supports HTML tags inside the tooltip
Lightweight and degradable
Extremely flexible & fast to set up
Easily styled with 100% CSS - no images needed
Plays nicely with IE
Developed by Caleb Jacob in 23 hours with 2 bag of Fritos and 7 cups of water. Feel free to use and abuse (or improve)!