OPENSTEP

From GNUstepWiki

OPENSTEP is a complete operating system that looks like http://www.linuks.mine.nu/openstep/

It is able to run on the original NeXT Hardware, SUN SPARC, HP PArisc and x86 computers.

You can find a copy here: ftp://ftp.sunwizard.net/pub/MiscOS/NeXT/OpenStep/

OPENSTEP was also available for on top of Windows and Solaris. You can find the one for Solaris here: ftp://ftp.sunwizard.net/pub/SunSoft/OpenStep/