One of the first secure telephony programs. Not limited to VOIP but can also directly encrypt a telephone line by using modems. Calls are encrypted using Diffie-Hellmann key exchange and several symmetric ciphers.
Download is possible from Germany only, see homepage
SIPNet is a p2p-sip framework, it support IM,audio,video based on p2p-sip protocols, and support file sharing like other p2p programs such as bittorrent. All SIPNet clients join to form a P2P overlay net and find each other using DHT algorithm.