Note: I wrote this article a while ago and this is a modified and expanded version.
Linux and most of the Linux applications are open-source
You can legally download, share, modify and use Linux applications because most of them are open-source and licensed under the GPL or another GPL-compatible license. This way, sharing is not prohibited like on Windows where you are not allowed to do it, so instead it's actually encouraged. Linux is all about free choices, from a freedom point of view, not price. You can sell software on Linux as well, and moreover, you can sell any GPL licensed application, even if you didn't create it.
Linux benefits from some wonderful projects like the GIMP, Apache, Amarok, OpenOffice.org, the kernel itself, all the GNU command line tools and other maintenance tools, entire desktop environments like KDE or GNOME, all for use freely and legally.
Hundreds of good distributions from which to choose from
A distribution is an entire operating system, just like Windows or Mac OS X. There are projects which take all the pieces of software out there, and together with the Linux kernel, put up an entire operating system, also called a Linux distribution, or GNU/Linux distribution. Such examples are Debian, Ubuntu, Mandriva, OpenSuSE, Fedora Core, Damn Small Linux, Slackware or Red Hat.
Linux was built with security in mind. Linux is not affected by viruses, spyware or other forms of malware software. Although some claim that there is no malicious software for Linux because it's not that widely spread as Windows, Linux by nature is more secure. It has a firewall integrated in the kernel and the fact that all of its components are open-source it helps developers to find security vulnerabilities fast and patch them. The code is there, opened for everyone to analyse it and eventually report if something is wrong. That is of course not the case with closed-source software, where security holes often remain undetected and opened for the first cracker to exploit them. The only security problem out there is constituted by users themselves, but this applies to any operating system. Never run commands which you don't know what they do and you'll stay safe.
Although the term free software means the code is available to legally study, modify and share it, you can sell the software at any given time, because the license allows it. Actually there are many people who take distributions, put them on CDs or DVDs and legally sell or mail it for money. But there are many good distributions which don't cost a thing, not even for downloading. Take Ubuntu, Debian or Fedora. And examples are countless.
Projects which are unmaintained can be forked
If a project is no longer maintained by its authors, then someone else can take the code and continue to develop it. Or fork it, to get something new, different. When Beep Media Player ceased to be developed, a new project, Audacious, was started, and it was based on the BMP code.
No patent restrictions
You don't have to worry about the legal aspect when you use open-source media files, like OGG Vorbis (a format similar to MP3) or FLAC (Free Lossless Audio Codec, used more and more over WAV due to smaller compression sizes). There is no fee you'll have to pay when using these formats, either for listening to music or in your software. Artists benefit from the OGG Vorbis codec as well.
Updated: Jul 13, 2008 (Created: Jun 12, 2008)