Announcing the release of Tippy, a brand new plugin for WordPress. Tippy gives the blogger a way to make a tooltip out of anything. The style of the tooltip itself is highly configurable (with more options to come in future versions) and the tooltip body can include pretty much anything.
The tooltip works through the tippy tag:
[tippy title="A new tooltip"]This is the body of my nifty <b>new</b>tooltip.[/tippy]
Read more, see it in action, and download it from the Tippy page