GNUstep provides an object-oriented application framework and tool set for use on a wide variety of computer platforms. GNUstep is based on the original ?OpenStep specification created by ?NeXT, Inc. (now Apple). GNUstep is becoming continually more stable and is used in a production environments at several organizations.
Official site: http://www.gnustep.org/
See also DebianGNUstep