[Ronbo]

Linux is a variant of UNIX the guy that wrote it based it on UNIX. He released it for free on one condition, any changes, improvements have to also be offered for free. (GPL)

UNIX was able to run on both Intel and Motorola CPU's unlike MS DOS.

There's a third OS that was the old VME (VAX) from digital computer that HP still maintains. This was the first high stability OS ever offered and there's a dedicated crowd of mainly ex-digital employees that run it. This was actually the first OS ported to the itanium processor. MS and Linux finally came out months later.

As far as I know HP still recommends VME for the high dollar servers. The next step up from these is a Cray.