Debian Linux

The third one is Debian Linux.

Debian is a Unix-like computer operating system and a Linux distribution that is composed entirely of free and open-source software, most of which is under the GNU General Public License, and packaged by a group of individuals known as the Debian Project. Three main branches are offered:
- Stable
- Testing
- Unstable
The Debian Stable distribution is one of the most popular for personal computers and network servers, and has been used as a base for several other linux distributions.
The Debian Testing distribution and Debian Unstable branches are rolling release and eventually become the stable distribution after development and testing (Unstable becomes Testing, and Testing becomes Stable).
Debian was first announced in 1993 by Ian Murdock, and the first stable release was made in 1996. The development is carried out over the Internet by a team of volunteers guided by a project leader and three foundational documents:
- Debian Social Contract
- Debian Constitution
- Debian Free Software Guidelines
New distributions are updated continually, and the next candidate is released after a time-based freeze.
As one of the earliest Linux distributions, it was envisioned that Debian was to be developed openly in the spirit of GNU. The vision drew the attention and support of the Free Software Foundation, which sponsored the project for one year. Upon the ending of the sponsorship, the Debian project formed the non-profit organisation Software in the Public Interest.

Debian has access to online repositories that contain over 43.000 software packages. Debian officially contains only free software, but non-free software can be downloaded from the Debian repositories and installed. Debian includes popular free program suck as LibreOffice, Iceweasel(Firefox) web browser, Evolution mail, K3b disc burner, VLC media player, GIMP image editor and Evince document viewer. Debian is popular choice for web servers.
The cost of developing all of the packages included in Debian 5.0 Lenny(323 million lines of code) has beed estimated to be about US$ 8 billion, using one method based on the COCOMO mode. As of 2014, Ohloh estimates that the codebase(78 million lines of code) would cost about US$ 1.5 billion to develop, using a different method based on the same model.
The current stable release, Debian 8.2 code-named Jessie, is officially supported on ten architecture ports. Notable changes in this release include using systemd as the default init system.

Debian supports two kernels, Linux and kFreeBSD, and offers GNU Hurd unofficially. GNU/kFreeBSD is released as a technology preview for IA-32 and x86-64 architectures, and still lacks the amount of software available in Debian's Linux distribution. There are several flavors of the Linux kernel for each port; for instance, the i386 port has flavors for IA-32 PCs supporting Physical Address Extension and real-time computing, for older PCs, and for x86-64 PCs. The Linux kernel does not officially contain firmware without sources, although such firmware is available in non-free packages and alternative installation media.

Pros:
- have three versions: stable, testing and unstable (you are not a permanently beta tester like on Arch)
- fast updates
- basis of user friendly Linux versions
- if you know what you want, configuration it is just more check boxes to fill in
- Debian does much better troubleshooting of new modules and anything you're working with
- does not have Amazon ads built into it
- very good security
- much stable and has better future than the Linux versions that are kept up by a group of fans, or one guy in his basement
- comes with a lot of different desktops (server and desktop versions)

Cons:
- it is harder to get much help from Debian user forums
- low customization

References:
https://wikipedia.org