The FS2 code (the engine) has been released under a fairly non-restrictive license. However, I would hesitate to call it "open-source," as that term has specific licensing requirements which the Volition source code license does not meet. Most open-source licenses allow you to sell a product based on that code, provided you make the source available; the Volition source code license prohibits that.
And the game itself -- the campaigns, models, graphics, etc. -- have
not been released at all under the source code license, so the only license that applies to them is the one that comes with the game.
In other words:
- you may distribute the source code, for free
- you may distribute executables, libraries, etc. for for free; AFAIK you are not required to redistribute the source along with it (a major distinction between the Volition source code license and the GPL)
- you may not charge for the source code, nor for anything built with the released source code
- you may not redistribute the original game -- executables, models, movies, etc. -- it is not covered under the source code license, and the applicable Freespace/Freespace2 license prohibits it.
Obviously, this last rule is bent a little: people release mods that contain TBLs (with original Volition data), and/or modified/re-skinned Volition models; this seems a little questionable from a strict IP perspective, but it is done all the time...
IANAL, but that's my interpretation of it. Do whatever you feel is right
