You use Firefox for Web browsing. You know it's a free Web browser that's safe, quick, and has all kinds of add-on modules (there are thousands of these — for chatting, bookmark management, social networking, image-processing, and even federal court-file browsing — at addons.mozilla.org). It has frequent updates to fix bugs, and every new version seems to find a new cool way to make the Web easier.
Thank Richard Stallman and the GNU project for all of those things. Apart from their programming skills, the genius of all their work is really the GNU General Public License (GPL), the legalese rubber where the free-software movement hits the intellectual-property-law road.
The GPL is one of several "copyleft" efforts, in which creators assert their copyright to something, but only for the purpose of ensuring that it — and any future works based on it — can always be distributed for free. (People can, and do occasionally, charge for their adaptations, but there's a disincentive: anyone who pays for it can, under the license, turn around and give it away for free themselves.)
It is legally different from placing a work in the "public domain," from which any person can take, repackage, and sell freely (that's how book publishers can reprint Shakespeare's plays, for example, and charge for copies). The GPL is a license to a user from a copyright holder, granting permission to use the material, but only under certain conditions (namely, free distribution of anything made with the material).
For example, while Firefox development is not coordinated by the GNU group, it uses some basic code that was first created by them. Programmers don't often want to bother creating the nuts and bolts — they want to make the machine. So they reach for the nuts and bolts, locate GNU-created free code, and find themselves in GPL-land, where all code is free, but all modification or adaptation of that code is also free. They are effectively enlisted in the free-software movement, even if their users don't know it.
Not all programmers want to start with GNU-created code (or something built on its foundations). Some actually do start from scratch and write all their own stuff. But the GPL is available to them, too — they just have to declare that.
It is the GPL that allows all of this — it is the key to the success, expansion, and future growth of the free-software movement. Courts in the US, Germany, and France have upheld the license's terms, requiring people who were charging for the software to stop doing so, and enabling out-of-court negotiations with companies that have also succeeded in affirming the GPL.
Many programs — even those on Windows or Mac platforms (against which Stallman rails) — are GPL-covered. The GPL is not only more widespread than GNU/Linux machines, but has the power to invade and co-opt those private platforms, making them more open over time, and showing developers and users alike the possibilities of open and free software.
Go to gnu.org/licenses/gpl.html for more information.