The developers released the source code and the public is taking advantage of it.  A Babylon 5 mod, Battlestar Galactica, Star Wars and even a Wing Commander saga?

First, you just find yourself a retail copy of the game (from like Good Ol Games) then run the installer which will download a ton of content and convert your game to FreeSpace Open.

If you want more info, Hard-Light Productions is a great place to start.