nextstep only runs on next hardware. osx is the evolution of nextstep so running it for anything other than entertainment purposes would be fruitless. i think there was a version of openstep that ran on top of solaris, and there's a free x86 version of solaris, but i think openstep requires the sparc version. if you did manage to find a machine to run it (even old next hardware is still pretty expensive) you'd still have to buy next/openstep, as it was never released for free.