Yahoo Messenger 9 has the following features (all available in Win XP) and part of them available in Vista, MAc or Linux:

Instant Messaging:
Send text messages in real-time to your friends on Yahoo! or Windows Live™ Messenger.
Chat Rooms:
Join a chat room to meet new friends while you discuss your favorite topics.
Photo Sharing:
Share photos from your desktop or Flickr, then discuss them over IM while you and a friend view them together.
PC-to-PC Calls:
Make a voice call to another Yahoo! Messenger user for free (microphone and speakers/headset required).
Phone Out (PC-to-Phone Calls):
Call friends on their regular or mobile phones from Messenger for as low as 1˘ a minute (Phone Out account required).
Phone In (Get a Phone Number for Messenger):
Get a new phone number that allows you to receive calls in Yahoo! Messenger (Phone In account required).
SMS (Text Messaging):
Send text messages from Messenger to your friends mobile phones for free.
Webcam:
Plug in your webcam to share live video with your friends on Yahoo! Messenger.
IM Conferencing:
Instant message with many friends at once in a conference room (includes voice capabilities, where available).
IM with Friends on Other Networks:
IM with friends who use Windows Live™ Messenger, Reuters Messaging, and Lotus Sametime — right from Yahoo! Messenger.
File Transfer:
Send files instantly to a friend while you IM (2 GB limit).
IM Forwarding to Mobile:
When you sign out of Messenger, have new IMs sent to your phone as text messages.
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.
Yahoo! Address Book:
View and edit your Yahoo! Address Book information for your contacts right from Messenger.
Stealth/Privacy Settings:
Make yourself appear online to some friends, and offline to others.
Call Forwarding:
Have incoming calls to Messenger forwarded to another phone number, even if you're signed out (Phone Out account required).
Yahoo! Mail alerts:
Get notified when new a new Yahoo! Mail message arrives.
Voicemail:
When friends call you on Yahoo! Messenger, they can leave you a voicemail if you're unavailable.
Message Archiving:
Maintain a private archive of your IM conversations.
Buzz Alert:
Get your friend's attention with a click of the Buzz button.
Yahoo! Updates:
Get real-time updates in Yahoo! Messenger about what your friends are posting online, reviewing and generally buzzing about
Plug-ins:
Add content, services and games to Messenger that you can enjoy on your own, or with friends while you IM.
Audibles:
Send an animated, talking character to a friend to liven up your IM conversation.
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.
Skins:
Choose a different skin to give your IM world a new look.
IMVironments:
Liven things up with interactive, themed backgrounds in the IM window.
Yahoo! Games:
Play a game of pool, backgammon, checkers and more with a friend while you IM.
Custom Status Messages:
Tell your friends what you're doing, seeing or feeling by customizing your online status message.
Custom Ringtones:
Assign ringtones to different callers, or upload your own audio files to use.
Customizable Fonts & Colors:
IM with a font, color and style that suits your personality.
Sound Effects & Soundtrack (During Voice Calls):
Throw a sound effect in while you're on a call or upload a music file to play as a soundtrack in the background.