NYT – If you signed up for Gmail (Google’s e-mail service) , you can use its Google Talk 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 messages to people in your contacts 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 voice mail for friends on your contacts list. A version of Google Talk is also available for BlackBerry devices. Link.