FS2_Open is the original FreeSpace 2 engine, released to the open source community by Volition in 2002. Ever since then, a dedicated team has been hard at work keeping the engine up-to-date, enhancing it, fixing the bugs, and porting it to non-Windows platforms.
Simply the best and longest lasting game I have ever played. Community makes it better day by day and doesn't let it die as it never should be allowed to.