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 as well. You can easily create fixed or offset tooltips with blinding or fade in/out effect. You can also define Hooking for the tooltips which allows you to place your tooltips anywhere in relation to your target elements. The concept is simple, you define two corners that you want to ‘hook’ to eachother. One on the target element, the other one on the tooltip. It is licensed under Creative Commons, feel free to use it on your projects.

Requirements: Prototype Framework
Demo: http://www.nickstakenburg.com/projects/prototip/
License: Creative Commons License