Perhaps the best example of this concept is the GNU Project (GNU - GNU's Not UN*X) of the Free Software Foundation (FSF). The object of this project (my words, not FSF's) is to produce a quality operating system, development tools, and other useful software for use on computers.
The software of this project is "protected" by something which has been known as a copyleft, officially known as the GNU General Public License and/or the GNU Library General Public License. GNU/FSF software must be distributed in such a way that users can get access to source code if they so desire. Often, the person supplying the software, supplies a binary (executable) program and also source code. Another common case is to just supply source code. This source code is packaged up in such a way that it is usually easy to produce an executable program. There is nothing stopping the person supplying the software from charging for the costs of distribution (materials such as disks or tape, or shipping and handling), or from making a profit on the distribution of the material. But the user must be able to access source code at their discretion.
A perception exists "out there", that something is of lesser value if it is free (not charged for), or is very inexpensive. Because of this, it appears that the "Free Software" bandwagon is about to re-engineer themselves as the "Open Source" (R) bandwagon. Hopefully this will get around the stigma attached to inexpensive things lacking value. Linux and GNU software certainly doesn't lack value or quality!
This possibility of charging people for distributing "free" software has lead a number of successful companies into the free software market. [The following is NOT an endorsement on my part, I have never paid for any of these companies products or services. I just know they exist.] Cygnus Support (http://www.cygnus.com/) is well known for its support of the GNU C compiler (and perl). The Perl Clinic (http://www.perl.co.uk/) is another supporter of perl. Linux has many commercial supporters: Caldera (http://www.caldera.com/) and Red Hat (http://www.redhat.com/) are a couple of well-known examples.
And then there are the legions of programmers who "hack" Linux (or BSD, or GNU/Hurd, or ...). Who will attack bugs, security problems, what have you, at the drop of a hat. If the problem is somewhat obscure and not very serious, not many people will volunteer their time to work on it. But serious, mainstream problems are often solved within a day or so. Let's see Microsoft get bug fixes out that fast!
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