Linux
From George Smart's Wiki
Linux is a UNIX like operating system, based on the Linux kernel. It is an alternative to Microsoft Windows, and Apple Mac OS X. It is my preferred operating system, though I have Mac OS X and Microsoft Windows on other machines. Both my Desktop and Laptop run Linux, based on Debian. I run Ubuntu (64-bit) on my Desktop and MintLinux (64-bit) on my MacBook.
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