Ventrillo

"Ventrilo is a proprietary voice program which allows users to talk to each other, similar to
a telephone or VoIP conference call, but is accessed via the internet. Ventrilo is a feature
rich voice conference solution and is widely used not only for gamers, but for businesses
also.

A microphone and speakers, or a headset, may be connected to each person's
computer if they wish to engage in voice conversation, although there is a
built-in text chat feature."