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.
GNUstep seeks to be source code compatible with Cocoa, it can thus be used to develop and build cross-platform applications between Macintosh (Cocoa), Unix and Windows.
GNUstep's main development language is Objective-C, but GNUstep is not limited to that.