General Use
Ubuntu/Kubuntu/etc

One of the most popular general-use distributions with one of the largest selections of software. Based on: Debian
Fedora

The most cutting-edge general-use distribution on a 6-month release cycle. It's cutting edge in two ways: new versions of software, and it uses new software before other distributions use them. Based on: Itself; Originated from: Red Hat
openSUSE

A stable, general-use distribution for everyone that excels in enterprise environments. openSUSE includes a few different defaults such as it's package manager and the KDE desktop environment. Based on: Itself; Originated from: Slackware
Debian

One of the most stable and best Linux distros in existence, with a large selection of software. It is often used as the base of many other distributions. Based on: Itself
Linux Mint

A highly-customized distribution that includes many features out-of-the-box that other distributions do not, including codecs. Linux Mint has such a large following that it's starting to look toward going in its own direction. Based on: Ubuntu, Debian
elementaryOS

A simple and elegant distribution which is entirely compatible with Ubuntu. Based on: Ubuntu
PinGuy OS

A general-use distribution that comes with a number of customizations to make an easier out-of-the-box experience. Based on: Ubuntu
PCLinuxOS

Another general-use distribution that combines technologies from different distributions to present an overall great experience. Originated from: Mandrake Linux
Gentoo

A general-purpose distribution that is centered around machine-specific optimization. Until recently, all software was compiled on the system rather than installing binaries. Not recommended for new users. Based on: Itself
Slackware

One of the oldest living distributions, Slackware allows plenty of configuration and is a great general-use distribution. It requires some knowledge of Linux and isn't recommended for new users. Based on: Itself; Originiated from: Itself
Mageia

A multi-purpose distribution which aims at creating an easy desktop computing experience. Based on: Mandriva
Minimal
Arch Linux

A minimal rolling-release general-use distribution where you install only what you want or need. It's highly cutting-edge and there are no default installations. Most customizations are done manually in text files. Based on: Itself; Originated from: Itself
Lubuntu
A lightweight Linux operating system based on Ubuntu but using the LXDE desktop environment in place of Ubuntu's Unity shell and GNOME desktop. Based on: Ubuntu
Manjaro Linux

A fast distribution that uses Arch Linux as a base but aims to be easier to configure and use. Based on: Arch Linux
Bodhi Linux

A great lightweight alternative for systems that can't run other desktop environments. Still very functional and relatively elegant. Based on: Ubuntu
Puppy Linux

A very small distribution that uses an extremely low amount of resources. Great for older systems or those that need every drop of power for their tasks. Based on: Itself; Originated from: Itself; Compatible with: Slackware, Ubuntu, others with Woof tool
Crunchbang

A very lightweight Debian-based distribution which uses the Openbox desktop environment as its default.
Tiny Core

An extremely minimal distro which you can build up yourself
Commercial
Red Hat Enterprise Linux

A commercial solution from Red Hat for a great enterprise system, both desktop and server. Costs of the distribution come from support packages. Based on: Fedora; Originated from: Itself
SUSE Enterprise Linux

One of two preferred enterprise systems for most Linux administrators. SUSE includes plenty of tools to make the admin's job a lot easier. Based on: openSUSE
Enterprise (Free)
CentOS

A great, free clone of Red Hat Enterprise Linux for a pure, stable enterprise system. Based on: Red Hat Enterprise Linux
Specialised Purpose
Hybryde Fusion

An Ubuntu-based distribution which lets you switch between desktop environments without restarting. Based on: Ubuntu
Joli OS

One of the best Linux distros for use on netbooks and working with different cloud services. Who needs ChromeOS? Based on: Ubuntu
ArtistX

A specialized distribution that includes virtually every single multimedia application for Linux that you can think of. Great for work on images, sounds, and videos. Based on: Ubuntu
GParted

A distribution made especially for partitioning hard drives. The distribution comes as a LiveCD only and contains only the GParted partitioning software and a console. Based on: Debian
Parted Magic

A distribution aimed specifically at partitioning your hard drives, but includes plenty of other features that make it more of a rescue disc than a lone partitioning tool.
Kali Linux

A fantastic distribution that is specialized for network penetration testing and security auditing. It includes all kinds of software and needed patches to do the job. Based on: Ubuntu; Originated from: Whoppix
For Mac and Windows Users
ZorinOS

A general-use distribution that mimics the look, feel, and functionality of Windows. This is geared for those who prefer the Windows interface or need an easier way to transition from Windows to Linux. Based on: Ubuntu