Most commly used linux distros.

Linux.org - Linux help and information about different distros.

Ubuntu - Most common and widely used Linux OS.
www.ubuntu.com/

Kubuntu - KDE desktop environment biased on Ubuntu.
www.kubuntu.org/

Xubuntu - XFCE desktop environment biased on Ubuntu.
xubuntu.org/

Lubuntu - LXDE environment biased on Ubuntu. Lightweight Ubuntu.
lubuntu.net/

Mint - Ubuntu compatible OS.
www.linuxmint.com/

Debian - Basic OS.
www.debian.org/

Kali - Previously known as BackTrack, Kali is used for offensive security.
www.kali.org/

openSUSE - Free OS also can feature other desktop environments such as XFCE and LXDE.
www.opensuse.org/

Fedora - Red Hat based OS.
getfedora.org/