Click on this link http://gmail.afraid.org/ and follow the instructions!
Stats since Oct. 4, 2004:

Google Talk’s been integrated into many Gmail accounts by now, and while it’s pretty neat, the chat popup that appears any time you rollover someone’s name could drive anyone batty.
Happily Google expert Garett Rogers provides a Greasemonkey user script that disables the popup without having to shut off the chat feature entirely. Useful! Free download, Firefox and Greasmonkey only.
Via | Lifehacker.com