GNUStep

http://www.gnustep.org/

 

The purpose of this project is to create a free and open version of the Cocoa (formerly known as NeXTSTEP/OpenStep) APIs and tools for as many platforms as possible.

GNUstep provides a robust implementation of the AppKit and Foundation libraries as well as the development tools available on Cocoa, including Gorm (the InterfaceBuilder) and ProjectCenter (ProjectBuilder/Xcode). GNUstep currently supports Unix (GNU/Linux and GNU/HURD, Solaris, NetBSD, OpenBSD, FreeBSD, Darwin) and Windows .