You can add things on top, like Red Hat and Canonical do with GUIs, but the code really isn't the users' to do with as they please.
Well, actually it is. Anyone can download the source code of the Linux Kernel (and other GPL'd code) and make changes to it, which they can either just use themselves or send it in as a patch for approval. Torvalds (he actually has a team with him, working on different sections of the kernel) can approve or not. But, and this is where the term "free as in freedom" really kicks in, if Torvalds doesn't approve the new patch, the submitter can always take the original kernel code and release his own version of it, with his changes intact (and there are always a few unapproved patches floating around for specific results, most notably the patch for low latency, which Torvalds doesn't want to implement). Or if Torvalds decides to quit or (god forbid) kicks the bucket, the kernel and its code is still there, and anyone can continue developing it (and there exist quite a few software projects out there that have been forked like that:
http://bit.ly/Lm68J). With proprietary software, if the company decides to kill it (or goes under), it stays dead, leaving the users out in the cold (imagine if FCP had been an open source software...).
Of course, the bigger the software gets, the more difficult it gets to organize, and that's where the bigger companies come in (Mozilla et.al.). I'm not sure what will happen with Lightworks, but I seem to remember them saying (some time ago, though) that they intend to release the core code, but that they are still cleaning it up and removing proprietary parts (the same happened with Netscape to Firefox, it took months before the first version of the source code was released).
