From TeamSpeak Systems:
TeamSpeak is software for quality voice communication via the Internet. Our platform offers a client-server architecture where client and server software is clearly separated. This means our server product runs as a dedicated server, rather than peer-to-peer. The TeamSpeak server software is literally capable of handling thousands of simultaneous clients or users, which results in an Internet-based teleconferencing solution that works for a repertory of different applications such as an alternative to conference calls, cutting long distance telephone costs, or just as a method of personal communication with friends and family. The software is free of charge to non-profit entities while commercial or for-profit entities have the ability to become part of our licensing program.