No Free Lunch, or Beer
One interesting aspect of the GPL and the notion of free software is that the term free in this context is often confusing. The Free Software Foundation and the open source movement are not necessarily about giving software away. Some opponents of the GPL have made the argument that it flies in the face of a free market, but that's not necessarily accurate. Certainly, some software vendors give away their products (or some version of their products) under the open source/free software banner. However, when we talk about free software and the GPL, the term free doesn't refer to cost.
There are many free distributions of the popular Linux operating system, which can be downloaded online for no charge. However, there are also companies such as Red Hat, which sells a version of the software and is a for-profit entity. For example, at the time of writing this article, you could download Linux for free online. Or you could purchase the SUSE Linux 9.0 Professional Edition for $59.99. Or the Red Hat Professional Workstation edition for $84.99. (These are just two of many possible examples.) Both products are Linux. But both versions have additional software and documentation added to the base Linux that's distributed under the GPL. How is this possible? We'll get to that later. For now, just remember that "free" doesn't mean "no cost." The FSF likes to say that "free" in the context of software should be thought of this way: "Free as in free speech, not as in free beer."