Yahoo! Messenger is a popular advertisement-supported instant messaging client and associated protocol provided by Yahoo!, that allows you to see when friends come online and to send them instant messages.

It also can alert you to new e-mail in your Yahoo Mail or Yahoo Personals accounts, or when you have upcoming events recorded in your Yahoo Calendar. Yahoo Messenger offers firewall support, a standby mode that minimizes the program until an Internet connection is made, the ability to save and print your conversations, and a tabbed interface that provides quick access to your favorite stocks, news, and sports scores.

Yahoo! Messenger is provided free of charge and can be downloaded and used with a generic "Yahoo! ID" which also allows access to other Yahoo! services, such as Yahoo! Mail, where users can be automatically notified when they receive new email.

Yahoo! also offers PC-PC, PC-Phone and Phone-to-PC service, file transfers, webcam hosting, text messaging service, and chat rooms in various categories.

Other features include the ability to create a chat room automatically, stock price alerts, the ability to send messages to friends (even if they're not online when you are), a Quick Compact mode that hides the Messenger tools to maximize your viewing area, Messenger Themes, and voice chat, which allows you to talk for free to anybody else on the Internet. It also includes Launchcast Radio, Yahoo Games, and Yahoo Audibles. Using a Webcam, you can see who you're chatting with or view the Webcam of any Yahoo member who has granted you access.

Yahoo! has announced a partnership with Microsoft to join their instant messaging networks. This would make Yahoo! Messenger compatible with Microsoft's .NET Messenger Service. It also made Microsoft's Windows Live Messenger compatible with Yahoo!'s Network. This change has taken effect as of 2006-07-13, - Yahoo! Messenger has integrated instant messaging with Windows Live Messenger users, and is fully functional.