Sure there is. It's part of many organisations' business models. To take one heavy-handed example, Google. They grab Linux-based source code, create their own internal distribution, and use it to power internal development and massive servers that are turning in a pretty penny.

I don't get it. You agree then that this license forbids me from grabbing someone else's free code, mixing it with NASA's, because it's not my original creation, but someone else's? How are you extrapolating from "your original work" to "paper trail"?

Huh, the GPL doesn't forbid you from keeping changes private. It forbids you from distributing changes without source. If you don't distribute, there is no problem.

Furtheremore, Whatever, GPL isn't the only free license. Use a BSD-style license or any other license without copyleft.

I still don't understand how an agency of the US government can claim copyright, though. Usually what happens is that the government subcontracts to individuals and are then bound by the copyright claims of those individuals. How is NASA getting away with this?

The problem is that the wording in the NASA license insists that the work must be your "original creation". You seem to not address the problem of how this wording seems to not allow you to grab someone else's code and combine it with NASA's to create a new work.

Why does NASA, a government agency, claim copyright on software?

And why does NASA release software under a non-free license?

It's not that hard. Use an existing license. Stop inventing your own licenses that conflict with truly free collaboration.

Oh, great, finally an udate to MAFIAAFire.... plus advertising for the seized domains! Now I want to check out what the US is censoring, and thanks to MAFIAAFire I can!

How do you know? I'm surprised I had to dig this deep in the comments to find out.

Wesnoth has some of the most beauutiful C++ out there (yes, there is such a thing as beautiful C++). If C++ is what you want to work with, I recommend you start looking at their stuff. Play the game first, of course, so you can start to get a feel for what sorts of things it does. Then you should be able to start guessing where things in the code may be. Step through the code with a debugger too, of course. I find that "ok, I'm gonna try to make the code do this", i.e. starting with a specific goal, setting breakpoints, and stepping through the code is the best way to get comfortable with an unfamiliar codebase, no matter its size.

MAME isn't open source, though. It forbids commercial use.