This is the simplest jQuery Tooltip plugin I could find. It is literally plug-and-play. Very simple. Just click on the links found below to view the different examples.
Note that you have to view the source code of the CSS and Javascript. But to give you and idea of how simple it is to use, you just do something like this:
<a href="http://cssglobe.com" class="tooltip" title="Web Standards Magazine">Roll over for tooltip</a> |