Some useful links:

TeamSpeak: http://www.goteamspeak.com/

Ventrilo: http://www.ventrilo.com/

Skype: http://www.skype.com/

Xfire: http://www.xfire.com

The all seeing eye: http://videogames.yahoo.com/multiplayer

miRC: http://www.mirc.com/

Msn: http://www.msn.com