Google Talk
Google's Chat Client came into the game very late, but it had a successful marketing strategy. Google Talk was released built into gMail, and became an extension of gMail. People were able to see which of their email friends (or business associates) were online at the time and quickly start a chat session - being more convienient and quicker than an email conversation.
For those who downloaded the seperate Google Talk program, they could do the opposite. Chat while keeping an eye on their incoming email.
Google Talk was a pretty featureless chat client at its beginnings, but Google programs usually are, and tend to add the bells and whistles as time goes on. I don't think it even had smilies until recently. It's best that the program stays clean and simple.