Yahoo! Messenger is a free instant messaging service that you can use to communicate with other people who also use Yahoo! Messenger.

Yahoo! Messenger Features
Entertainment:

LAUNCHcast Radio - New!
Listen to LAUNCHcast Radio. Make your own radio station and listen to the music you like! Your online status message shows people what you're listening to.

Yahoo! Games - New!
Challenge your friends to classic Yahoo! Games in the IM window. Games include Word Racer, Checkers, Chess, Dominoes and more. See your friends' high scores for selected one-player games.

IMVironments
Customize your IM window with interactive themed backgrounds. See the IMVironments
Personalization:

Audibles - New!
Send people these talking animated characters to say hello, goodbye, flirt or taunt your opponent during a game.

Emoticons
Express your feelings in a conversation using these little animated smiling faces.

More....