There are literally hundreds of Linux distributions out there (Red Hat, Debian, Ubuntu, etc..). Feel free to add your favorites!
Univention Corporate Server (UCS)
UCS is an Enterprise Linux IT infrastructure management solution across different platforms
A modern, elegant and comfortable operating system which is both powerful and easy to use.
Freedom. Friends. Features. First.
Fast, free and incredibly easy to use
The Oz Unity Linux distrubution. Made by the international Oz Unity remix team.