An telephone/voice conference program that works over TCP/IP (Ie, it's one of those "Internet telephone thingies" people speak of. =)
It supports variety of voice compression systems (GSM, ADPCM, and so on), and cryptography (IDEA, DES, PGP).
It's more or less public domain, and available for Windows and *NIX.
http://www.speakfreely.org/