Mouse over or rollover effects reveal interface clues to users.
You can download the ToolTip script to have your tips like the ones on this page.
The links lead to pages demonstrating different effects using events and JavaScript. These use the onMouseover and onMouseout events.
Note: these scripts work on Netscape 6 and IE 5 (and greater), Mozilla, and Opera 7.21. They were tested on Konqueror 3.05 and Opera 6.02 and did not worked correctly.