GNUstep is a free software implementation of NeXT's OpenStep Objective-C libraries (called frameworks), widget toolkit and application development tools for Unix-like operating systems. It is part of the GNU project.
GNUstep features a cross-platform, object-oriented development environment based on and completely compatible with the OpenStep specification developed by NeXT (which has since been bought by Apple Computer). Like Apple, GNUstep also has a Java interface to OpenStep, as well as Ruby and Scheme bindings. The GNUstep developers plan to track future changes to Apple's Cocoa to remain compatible.
History
GNUstep began when Paul Kunz and others at SLAC wanted to port HippoDraw from NeXTSTEP to another platform. Instead of rewriting HippoDraw from scratch and reusing only the application design, they decided to rewrite the NeXTSTEP object layer which the application depended on. This was the first version of libobjcX. It enabled them to port HippoDraw to Unix systems running the X Window System without changing a single line of their application source. After the OpenStep specification was released to the public in 1994, they decided to write a new objcX which would adhere to the new APIs. The software would became known as "GNUstep".