GNUstep is a Linux port that came from NeXTSTEP in the early 1990's. Steve Jobs had just figured out that the NeXT computer company was going to go out of business, so he open-sourced the software to his product. Eventually, an application was needed out of the software by some linux developers and was then ported to Linux.
Now, the GNUstep specifications are being made, and the developers at Linux have ported a free Mach kernel in instead of the one that was originally included in the NeXTSTEP package.
The GNUstep can run inside any Microsoft Windows machine, plus on it's own, for whoever misses the old NeXTSTEP system if they had one in the '80s or '90s. The development process may also allow for one day to be merged into a better graphics system, and is in the process of eveolving the application creation process. The OS is currently being made for more modern systems, including netbooks with Windows 7.