Pidgin is an easy to use, multi-protocol chat client client used by millions. Connect to AIM, MSN, Yahoo, XMPP, and more chat networks all at once. It has built-in support for NSS, offering client-to-server message encryption for protocols that support it.
The goal of Gajim is to provide a full featured and easy to use XMPP client. Gajim works nicely with GNOME, but does not require it to run. It is released under the GNU General Public License.
BeagleIM is a lightweight and powerful XMPP client developed by Tigase, Inc. It provides an easy way to start using the XMPP Protocol (formerly known as Jabber) if you've never used it before. Veterans of the protocol will find many features here they are familiar.
Vacuum-IM is a crossplatform Jabber client. The core program is just a plugin loader - all functionality is made available via plugins. This enforces modularity and ensures well defined component interaction via interfaces.Discontinued
The project seems no longer developed. Last version, 1.2.5, released in June 2015, can be still downloaded from GitHub