MSN Messenger
With MSN Messenger you can chat online via text, voice or even video conversation - in real time - with your friends, family or colleagues. It's faster than e-mail, a great choice for conversations and the perfect alternative when you can't be there in person. And, oh by the way - it's FREE.
Download

Skype
Skype is a simple bit of software they want to give you. It’ll let you make free calls to your friends all over the world. And they don’t want any money for it. It’s free, and easy to use.
Download

GAIM
Gaim is a FREE multi-protocol instant messaging (IM) client for Linux, BSD, MacOS X, and Windows. It is compatible with AIM and ICQ (Oscar protocol), MSN Messenger, Yahoo!, IRC, Jabber, Gadu-Gadu, SILC, GroupWise Messenger, and Zephyr networks.
Gaim users can log in to multiple accounts on multiple IM networks simultaneously. This means that you can be chatting with friends on AOL Instant Messenger, talking to a friend on Yahoo Messenger, and sitting in an IRC channel all at the same time.
Download

Mozilla Thunderbird
Thunderbird makes emailing safer, faster, and easier than ever before with the industry's best implementations of features such as intelligent spam filters, built-in RSS reader, quick search, and much more. An excellent FREE email client.
Download