From GameSpy Industries:
Roger Wilco is a real-time voice chat technology that enhances communication among users of the internet. Use it to talk to your friends, play multiplayer games, or add a voice chatroom to your site.
Roger Wilco is a stand alone application. You won't need a second soundcard - just one capable of full duplex audio. A microphone and a 28.8k modem gets you in the door!