ACM (flight sim)
ACM is an open source flight simulator for UNIX, and with a older release for Windows 95 and Windows NT. The Windows version was released in 1997 by Web Simulations, and supported an advanced networking feature called Distributed Interactive Simulation (DIS).  ACM included two aircraft, a F-16 Falcon and Mig 29 and included multiplayer, HUD, and joystick support among other features.
The latest UNIX version is version 5. System requirements in the 1990s included 120 mhz pentium, 16 Mb ram, and 6 Mb hard drive space. Recommended screen was at least 800x600 resolution and 256 colors. 
ACM was examined during the development of FlightGear in the 1990s, as mentioned in Flight Gear - just another Flight Simulator? by Dr. Michael Basler, Jena.
"Meanwhile, work is continuing on other, related modules. Curtis Olson (University of Minnesota, Minneapolis) has disassembled the freely distributed UNIX military flight simulator ACM and modified certain core components for our purposes. I have put together an installation guide. Further information on the developers, the modules, the current state of development, the overall concept of the project and much, much more can be found on the Flight Gear homepage:" 
Available on the FlightGear website here: http://www.flightgear.org/Papers/Basler-1997/FSFD1_97_trans.doc