wz is a MediaWiki extension I wrote to enable tooltips (or mouse rollover).
Use the wz tag with an argument to tip that will be displayed over the encapsulated element:
<wz tip="That's ''that'' simple!">Roll over me!</wz>
Roll over me!
You can defer the tip to a tag using tagtotip instead of tip:
<wz tagtotip="tip">Roll over me!</wz> <span id="tip">That's ''that'' simple!</span>
Roll over me! That's that simple!
This way you can make very complicated tooltips. In particular, you can wikify them! (including images, etc. For links, you'll need to wait that I implement the sticky option).
You can conveniently "tooltip" all sorts of things, such as images:
<wz tip="That's Walter on a quick 200km trip with his homebuilt carbon lowracer. This guy is awesome.">[[Image:Kreuzot_ss.jpg]]</wz>
echo Html::linkedScript( '/extensions/wz/js/wz_tooltip.js' );