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Linux was written from ground up as an alternative to UNIX by Linux Tuvalds/ing (author of a really nice OS book which incorporates the full source code for linux) as a modular os which could be worked on even in a mainframe environment without taking down the whole system -- to the point of testing new device drivers in a virtual machine. Itis a nice, small, clean, integrated piece of work. FreeBSD, NetBSD, etc are all ports of BSD UNIX 4.2/4.3 (thereabouts) which are really compilations of collaborative works, possibly under question as to the proper copyright owner -- one commercial company took on AT&T in a court case to establish the right to market 386/BSD, BSD/386 (I can never recall which one is commercial and which is gnu licensed) - but as I undetstand it, their win in court only applied to BSDI Inc and did not clearly resolve the copyright issue for everyone else. If you play with NetBSD, 386BSD source code that may be OK -- but if you market it AT&T's clients (sold to Novell? resold to SCO?) may have a basis to come after you. Linus owns linux outright.