GNU ACM is a distributed aerial combat simulator that runs on the X Windows System. Players can engage in simultaneous air combat from different computers. Players fly jet aircraft (something close to either an F-16C Falcon or a MiG-29 Fulcrum) equipped with radar, heat seeking missiles and cannon.
GNU ACM is a slighty modified version of the original ACM made by Riley Rainey at WebSimulations.
Finally, GNU ACM is free software. This means that everyone may use it, redistribute it and/or modify it under the terms of the GNU General Public License, as published by the Free Software Foundation (see the file COPYING that came with GNU ACM, for details).