Prototip allows you to easily create both simple and complex tooltips using the Prototype javascript framework. If you also use Scriptaculous you can even add some nice effects to them.
"Recently I had a project that demanded a rollover image preview. You know, one of those tooltip-like bubble popups that appears when you roll over link or a thumbnail. Since we were already using jQuery on that project I decided to take it easy and investigate what can be done with that extraordinary library."
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