Free Operating System

There are Operating Systems (OS) available in market with Linux / Unix Flavours.

There are also servers available for free.

Free Desktop OS:

Ubuntu - http://www.ubuntu.com/desktop

Debain - http://www.debian.org/

Free Server OS:

Ubuntu - http://www.ubuntu.com/server

Smart Phone / Tablet:

Ubuntu - http://www.ubuntu.com/phone        http://www.ubuntu.com/tablet

Android - http://www.android.com/