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.

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.
Yahoo! Messenger was originally launched under the name Yahoo! Pager on 1998-03-09.

Entertainment
- Photo Sharing. Drag-and-drop photos into your IM windows and talk about them like you're there.
- LAUNCHcast Radio. Make your own radio station and listen to music you like!
- Yahoo! Games. Right in your IM window, challenge friends to a game of backgammon or pool while you catch up.
- IMVironments. Use interactive themed backgrounds in your IMs.

Productivity
- IM Windows Live™ Messenger friends - New! Share IMs (and more), plus see them on your Messenger List.
- Yahoo! Calendar. Stay organized and get calendar alerts.
- Contact Search Bar. Quickly find a contact to IM, call, SMS and more.
- Find, Add, Share Friends. Easily get connected with the people who matter most.
- Yahoo! Search. Find what you need right from your IM window.
- File Transfer. Share files with friends instantly.
- Address Book. Synchronize all your contact information.
- Stealth Settings. Make yourself appear online to some and offline to others.

Personalization
- Plug-ins - New! Add content, services and games to Messenger.
- Sound effects - New! Play fun sounds or a song during PC calls.
- Audibles. Let these animated characters say it out loud for you!
- Emoticons. Express your feelings with these animated smiling faces.
- Yahoo! Avatars. Represent yourself with a stylized graphical image.
- Display Images. Select an image to represent yourself to your friends.
- Skins. Select new visual designs or disable them altogether.

Communication
- Chat Rooms. Join a live, group chat room to discuss a variety of topics.
- Make Calls From Your PC. Make free worldwide PC-to-PC calls, plus free voice mail.
- PC-to-Phone Calls* - New! Call regular and mobile phones for as low as 1¢/min.
- Phone-to-PC Calls* - New! Get a phone number for your PC so friends can call you.
- Connect on Yahoo! 360° - New! Stay up to date with friends' blogs, photos, and more.
- Take Yahoo! with You. Send and receive IMs on your mobile phone.
- Webcam. Share live video with friends and family around the world.
- Conference. Exchange text messages with multiple people in one IM.

NEW in Yahoo! Messenger 9 :
- Your friends come first. Enjoy a fun new look, with more room for friends' images, info, updates.
- Express your true self. Add your personality with new Emoticons, plus new skins.
- Share your passions. Use Flickr to swap photos (and always keep all your friends in the picture