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

Miranda IM is an open source multiprotocol instant messaging application, designed for Microsoft Windows.

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

Tkabber is a GPL instant messaging client for the XMPP protocol which uses the Tk toolkit for the GUI.

Trillian is a proprietary multiprotocol instant messaging application.

Miranda NG is a successor of Miranda IM, a popular multi-protocol instant messaging client for Windows.

qutIM is free and open-source multiprotocol (ICQ, Jabber, Mail.Ru, IRC, VKontakte) instant messenger.