Pidgin is a free and open-source chat client which supports AIM, Google Talk, ICQ, XMPP protocols.

Instantbird is a cross-platform instant messaging client based on Mozillas XULRunner and the open source library libpurple used in Pidgin.

Coccinella is a free and open-source cross-platform client for the XMPP/Jabber-instant messaging-protocol.

Jitsi (formerly SIP Communicator) is a free and open source multiplatform voice (VoIP), videoconferencing and instant messaging application.

Kadu is a free and open source multi-platform instant messaging client that supports Gadu-Gadu, XMPP IM networks and other services based on XMPP like Google …