Psi is a cross-platform powerful XMPP/Jabber client (Qt, C++) designed for the Jabber power users. Psi+ is a development branch of Psi IM XMPP/Jabber client.
Developed by http://psi-plus.com/
Apps available for
Mac OS X
Psi+ is an Open Source Product.
http://psi-plus.com/ has developed Psi+