Web site: www.bosrup.com/web/overlib/
overLIB is a large, feature-filled library for producing ToolTips. It can be used for anything from standard Tool Tips to pop-up message boxes. A plug-in API is provided, and plug-ins are available for tasks such as moving the pop-up message box while scrolling the page or adding drop shadows to the pop-up message box itself. overLIB contains a huge number of features, making it suitable for many tasks, but it can be overkill if you are adding just a few ToolTips to a site.