На английском, но думаю, все прозрачно и понятно:
Web users love informative clues. Whatever questions and misunderstandings might occur — delivering precise answers immediately is the primary task a responsive user interface should be able to cope with. To do that, developers have to consider subtle and well-thought tooltips — used correctly, they can greatly improve user experience and help users to get things done. In Web such “responsive” hints can be provided by tooltips. E.g., unclear input fields in web forms are perfect examples of a situation you might be willing to use a tooltip for.
We’d like to thank Jurgen Koller for compiling an extensive list of tooltip scripts we’ve stumbled upon during our search. It gives many useful pointers, but we’ve managed to find some more. You might be willing to use Koller’s post as a quick reference for your search.