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