Linux is an open source, free operating system. It is licensed under
GNU General Public License. To clarify, there are many
distributions (distros) of Linux, and most are free.
Some are free, but come with support from paid staff and bundled applications if you buy it. For example,
Mandriva Linux.
Sun may have made a distribution, and decided to not release it for free (ncnd), but that doesn't happen often. Usually,
Linux distros can be modified an distributed freely.
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