GNUstep

GNUstep is the Free implementation of the OPENSTEP specification originally designed by NeXT.

On NeXT being purchased by Apple, they made slight modifications to their implementation of OPENSTEP classes, and called it Cocoa. The code is essentially compatible, but the build systems are not.

GNUstep applications are created using the Objective-C programming language, using the GNUstep implementation of these classes.