Ventrilo is a high quality voice chat server that is very ideal for games and clans. It is free for clients though there is a cost for hosting the server.
It is very commonly used in World of Warcraft, Counterstrike and basically any online game that requires teamwork. Call it a improved, gamer friendly version of Skype which supports
1. Multiple Channels
2. Administration Rights
3. Up to 8 concurrent users (licensing reasons)
4. A variety of high quality yet low bandwidth codecs including GSM and Speex
You may find out more about Ventrilo at http://www.ventrilo.com and you can also download the client from there for free.
Edit: This service is for free! 😀