Windows users can download the Google Talk application, and Mac and Linux users can use many other instant messaging applications with their Google Talk accounts or message directly from Gmail. Google Talk also supports PC-to-PC voice calls.
When Google Talk is used from within Gmail on either Mac or Windows, you can also use it for video chats. The downloaded client does not support video as of this writing.
Google Talk can be downloaded from the Web at www.google.com/talk.