Gale is instant messaging software distributed under the terms of the GNU General Public License.

Several features set Gale apart from other instant messaging systems.
Gale is open source software.
The GPL ensures that you and others retain the freedom to modify and distribute the Gale source code. Gale will never lock into any one vendor's proprietary, closed system.
Gale is useful.
Gale isn't just about poking "private" messages to someone sitting at another computer. Gale does support secure private messaging, but Gale also has a well-developed infrastructure for public (and semi-public) chat. Advanced categorization and filtering features mean that you can precisely control your level of participation and distraction. We've been at this for years, we've tried everything else out there, and we have a lot of experience with the usability of real-time messaging systems. The result of our experience is something like IRC, something like Zephyr, and something like commercial "instant messaging" systems, but with many features you won't find in any of these.
Gale is secure.
Most other systems depend on the security of a central bank of servers, and provide no protection against network eavesdroppers. Gale uses strong cryptography for both privacy and authentication, and is designed to work in an environment of mutual distrust between users and administrators.
Gale scales.
Gale's architecture uses a loosely-connected set of servers which locate each other via DNS only when they need to talk to each other. Multicast is accomplished by the dynamic creation of self-healing spanning trees of interconnected servers. The network is robust; servers and clients detect and route around failure. This means Gale is fast and stable. Gale will not suffer the kind of performance and reliability problems USENET, IRC, and centralized commercial message systems do.
Gale is here today.
Gale has been in active development for over three years. Both clients and servers have been well tested by daily use in an active user community. Both simple command-line and sophisticated graphical clients are available, and there are platform solutions for the POSIX, Microsoft Windows, and Java platforms.

http://www.gale.org/index