Zen Dots is a time line tool that lets you efficiently view a timeline and associated data. As the viewer, you can expand or contract the scale of the timeline and drag the timeline to view various event descriptions. As the creator, you can set the dates, the colors, and the descriptions which are HTML.
Download the zip file for all you need to create your own timelines. It uses an XML configuration and data file. Alternatively, the tool, less the HTML output field, is available as a Flash component so you can use the timeline slider as an interface to control different types of descriptions.