Ubuntu and it's variants are an easy to install and use alternative to other proprietary operation systems. This is a Long Term Support (LTS) release and will be supported for 5 years.
Ubuntu - Unity Desktop Environment
Kubuntu - KDE Desktop Environment
Xubuntu - XFCE Lightweight Desktop Environment for older/slower machines
Mythbuntu - Setup a MythTV DVR
Ubuntu Studio - Distribution targeted at Audio Video and Graphics
Lubuntu - LXDE Lightweight Desktop Environment for older/slower machines