peak Freely is a program that allows two or more people to conduct a real-time voice conference over the Internet or any other TCP/IP network. It supports a variety of compression protocols, such as GSM, ADPCM, LPC, and LPC-10. The cryptography-enabled version includes IDEA, DES, and limited PGP encryption capabilities for protecting the privacy of important voice conversations.

http://www.speakfreely.org/