A long time ago, Apple bought NeXT and its operating system NeXTSTEP would later evolve into Mac OS X. GNUstep is an open source implementation of the NeXTSTEP APIs that was created even longer ago. Applications based on GNUstep run on many Unixes and (with some hassles) on Windows. I’ve followed it since its beginning, because I wanted to have the quality of NeXT’s APIs on an open operating system. Alas, GNUstep took a long time to mature. During this time, Mac OS X made it much cheaper to try out NeXT’s technology and my focus shifted towards web applications, so I never tried it out.
[Source: Daring Fireball]