XChat

XChat is an IRC chat program for both Linux and Windows. It allows you to join multiple IRC channels (chat rooms) at the same time, talk publicly, private one-on-one conversations etc. Even file transfers are possible.

mIRC

mIRC

mIRC is a popular Internet Relay Chat client used by millions of people, and thousands of organizations, to communicate, share, play and work with each other on IRC networks around the world. Serving the Internet community for over a decade, mIRC has evolved into a powerful, reliable and fun piece of technology. You can download mIRC here.

SKYPE

 Together, wherever you are Free calls, video calls and instant messaging over the internet. Plus great value calls to phones anywhere in the world.

 

Yahoo! Messenger 10 BETA

UPDATES!!

 

Enhanced video calling

Feel like you’re there with high-quality, full screen video and voice calling

Keep up with what your friends are doing

See your friends’ latest updates from Yahoo!, Flickr, Twitter and more with the new “Updates” view

Improved language support

Change your preferred language in Yahoo! Messenger with just a few clicks

source: http://messenger.yahoo.com/winbeta

YahooMessenger 9 Beta

Easier ways to stay up to date:

See more updates from your friends in the detailed Messenger list

Show off your status:

Easily add links and emoticons to your status messages

Watch videos together:

Share videos from popular sites and watch them with friends right in your IM window

source:http://messenger.yahoo.com/win/