Linux

An open source clone of the UNIX operating system, and by far the most popular of such. Technically speaking only the system kernel is supposed to be called Linux but it has become customary to refer to any OS distribution that is based on it and has a UNIX style userland by the name.

The Linux kernel is also used by a few operating systems that do not have a UNIX style userland on top of them, the best known of these is Android
 * The Linux kernel

Porting issues
See: Porting from Unix platforms

Linux distributions

 * Debian
 * Knoppix
 * Damn Small Linux
 * Sparky Linux
 * Gentoo
 * ChromeOS
 * Korora
 * Linspire
 * Mandrake
 * Mandriva
 * Mageia
 * PCLinuxOS
 * Mint
 * Oracle Linux - Red Hat fork
 * Red Hat
 * Fedora
 * CentOS
 * Scientific Linux
 * Slackware - Ruled the roost in the mid 90s but has declined since. Very UNIX like.
 * SUSE
 * OpenSuse
 * Gears on Gallium
 * GeckoLinux
 * LinKat - Government supported Catalan distribution.
 * Ubuntu - Currently the most popular distribution.
 * Elementary OS
 * Kbuntu
 * Lubuntu
 * Peppermint Linux
 * Yggdrasil - The first Linux distribution.