Free half duplex, channel based voice chat software. Supports Windows and Linux platforms... 

http://www.teamspeak.com/

Allows you to instant message with anyone on the Yahoo server. Tells you when you get mail, and gives stock quotes... 

http://messenger.yahoo.com/

The official Web-based version of MSN Messenger. Messenger is really about people - so the web version is now built into Hotmail and the rest of Windows Live... 

http://webmessenger.msn.com/

Not-for-profit organization that oversees the general development of XMPP and maintains the Jabber Enhancement Proposals... 

http://www.jabber.org/

Welcome to ICQ home, the place to download ICQ, take part in our multilingual community through blogs, forums, and free online games. You can also meet friends, find a date, chat in our Chat Rooms, and send Greeting Cards - check it out!.. 

http://www.icq.com/

Chat with friends and family on the internet using Google Chat.. 

http://www.google.com/talk/

A chat client for Windows that allows you to simultaneously connect to AIM, ICQ, IRC, MSN and Yahoo! Messenger... 

http://www.trillian.cc/

A modular open-source multi-protocol instant messenger for Windows that maintains a database of community developed plug-in's that add functionality... 

http://www.miranda-im.org/

The official Web site of AOL Instant Messenger. Exchange messages with friends, family, and colleagues online... 

http://www.aim.com/