For non-techies, the "OS" of the title stands for "operating system," the hidden, underlying software that enables your computer to do what computers do: run programs. Without an operating system, a computer is just a box — so it goes without saying that whoever owns the rights to the dominant OS pretty much rules the personal computer roost. Case in point: Bill Gates and Microsoft, whose Windows operating system now runs an overwhelming majority of office workstations and home computers. Microsoft not only owns the system — ever the capitalist, Gates led the way in the notion of "proprietary software" — but keeps the source code firmly under lock and key, which makes it pretty much impossible for anyone else to adapt the code to suit his or her needs. But it doesn't have to be this way. J.T.S. Moore's surprisingly exciting documentary chronicles the history of the Free Software Movement, an outgrowth of the utopian vision of free-code guru and überhacker Richard Stallman, who left MIT to dedicate himself to the collaborative development of non-proprietary software unfettered by intellectual property rules and regulations. In 1984, Stallman and hundreds of programmers and hackers linked by the Internet set out to create a copy of the UNIX operating system that could be downloaded, adapted, improved on and, most importantly, shared — for free. The result was the GNU project (often called LINUX, though Finnish programmer Linus Torvalds provided only one — albeit crucial — piece of a very large puzzle), which produced an OS faster (and cheaper) than UNIX-based systems and served as an innovative development model that ran counter to everything Microsoft was about. So what happened? The fascinating story Moore documents involves the battle between Microsoft and Netscape over the browser market and Netscape's decision to release its source code; the more practical, commercially minded "Open Source" movement the Free Software Movement spawned; and something called Windows Refund Day. Moore's film is unusually sharp looking for this sort of documentary, and comes complete with a nice soundtrack. But most important, it's as comprehensible as any "Dummies" guide, something even non-techies can enjoy.