Process-one is a company specialized in high-performance enterprise messaging solutions. The company provides solutions for the XMPP and SIP protocols.
SIP Communicator is an audio/video Internet phone and instant messenger that supports some of the most popular instant messaging and telephony protocols such as SIP, Jabber, AIM/ICQ, MSN, Yahoo! Messenger, Bonjour, IRC, RSS and soon others like IAX.
The Landell is an Instant Messenger and VoIP application written to fit well with the GNOME Desktop Environment. It is written in C and C# and it uses the Tapioca framework which is based on Telepathy.