Gmail is more than just mail

IF YOU signed up for Google's e-mail service, you can use its Google alk network to send instant messages to people on your Gmail address book.
You may have already noticed the Chat area in the Gmail window when checking your messages on the Web.
You can write message to people in your contact list right there in the browser and even save your transcripts to your Gmail account.
Windows users can also download stand-alone Google Talk instant-message software, which lets you transfer larger files between computers, make free PC-to-PC voice calls over the Internet and even leave mail for friends on your contacts list.
Mac and Linux users have other choices as well. There is a Flash-based version of the program called the Google Talk Gadget that works on most systems.
Mac OS X users can also connect and gab with Apple's iChat instant messenger program. Google has a page devoted to getting started with Google Talk at www.google.com/talk/about.html; there's also a link to a list of other IM programs that work with the system.
Because Gllgle Talk uses the Jabber network (www.jabber.org), your instant messagers don't go through the same network and servers used by chat sessions in the Microsoft Windows Live Messenger, Yahoo! or other instant messaging services.
It is possible to chat with people on other IM networks, usually by using IM software that link the different systemsl Psi, Adium and Trillian Pro are some of these "interperable" instant-messaging programs. - New York Times Special Features