Linux
Linux (or, rarely, GNU/Linux) originated as a free clone of Unix created by Linus Torvalds and a group of programmers communicating across USENET in the early 1990s. The version 1.0 kernel was ready in 1994, and it is currently at version 3.17. It was released under the GPL, which requires anyone using Linux as a basis for a new distribution to make the source code available, freely usable, and freely modifiable. Linux has gained extensive penetration into the server market, where it is particularly popular as the basis for web servers. The Linux kernel is also the foundation for Android, the most popular mobile operating system in the world.
[edit] Popularity
It is also popular among computer enthusiasts, but it has not achieved more than 2% penetration into the desktop market share, because few manufacturers bundle Linux with their computers due to agreements with Microsoft to pay a certain amount of money per machine in order to qualify for OEM pricing for their Windows licenses. Microsoft uses this monopoly power to threaten manufacturers with higher prices if they ship any PCs with Linux pre-installed. However, with the introduction of so-called netbooks (small, cheap computers - "SCC") by many manufacturers, Linux is finding a new niche in the computer market that cannot be catered to by Windows 8, given the latter's insistence on a confusing, contradictory user experience (i.e. the Start Screen) that nobody asked for. In fact, Linux is responsible for the prolonged life of Windows XP, after Microsoft belatedly realized the threat to its monopoly position posed by Linux.
Due to the lack of online activation, it is difficult to tell which distributions are the most popular. Most sources agree that either Ubuntu (a fork of Debian), or Linux Mint (a fork of Ubuntu) is the most popular.
[edit] Lawsuits
In 2003, the SCO Group, which licensed Unix systems on behalf of Novell, filed infringement lawsuits against IBM, Daimler-Chrysler, Red Hat, and Autozone for using Linux (Red Hat has its own distribution of Linux), which they asserted was derived from intellectual property they claimed to have created. They also piqued the interest of the Linux community at large by asserting a $699 fee for every installed copy of Linux, anywhere. Five years later the SCO Group filed for bankruptcy, impoverished by their own legal fees pursuing the litigation and defending against counter-claims. SCO's assertions were rendered moot by a judgment that the intellectual property was in fact held by Novell all along. Novell, which has its own Linux distribution, openSUSE, does not share SCO's beliefs.
Novell in turn wants whatever remains of the $24 million Microsoft paid SCO to run the whole scam, which was nothing more than a campaign to create "Fear, Uncertainty, and Doubt" (FUD) to discourage switching from Windows to Linux, while the Windows Vista rollout was delayed for years.