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

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.