Connect with all your friends:

Instant message with friends on Yahoo!, Windows Live™ Messenger 

Stay in control... 24/7:

Decide who sees you online with stealth settings

Share away:

Swap photos and monster files (up to 2GB) in real-time. Plus enjoy voice calls and webcam video 

System requirements: Microsoft Windows XP, Vista.

NEW! Try the all-new Yahoo! Messenger 9.0 Beta

Other versions: XP (9.0 Beta), Vista, Mac, Web, Mobile

Web

Connect with friends from any web browser — no download required.

Mobile

Stay connected with your IM friends right from your mobile phone.

Forward IMs to Phone

Keep the chat movin

Forward IMs to your phone and keep in touch from anywhere

Features

Communication

Instant Messaging: Send text messages in real-time to your friends on Yahoo! or Windows Live™ Messenger.
 
SMS (Text Messaging): Send text messages from Messenger to your friends mobile phones for free.
 
IM with Friends on Other Networks: IM with friends who use Windows Live™ Messenger, Reuters Messaging, and Lotus Sametime — right from Yahoo! Messenger.

Productivity

Contact Search Bar: Quickly find a contact to IM, call, SMS or more.
 
Yahoo! Search: Start a web search right from your Yahoo! Messenger window.
 
Stealth/Privacy Settings: Make yourself appear online to some friends, and offline to others.
 
Message Archiving: Maintain a private archive of your IM conversations.
 
Tabbed IM Windows: Reduce desktop clutter by organizing multiple conversations into a single window.

Fun & Personalization

Emoticons: Express your feelings with these animated, smiling faces.
 
Avatars: Represent yourself with a stylized, graphic image where you can choose the hair, clothing and more.
 
Display Images: Display an image to represent yourself to your friends.
 
Custom Status Messages: Tell your friends what you're doing, seeing or feeling by customizing your online status message.
 
Customizable Fonts & Colors: IM with a font, color and style that suits your personality.

Source: Yahoo! Messenger - Chat, Instant message, SMS, PC Calls and More