From the Apple site: Cocoa is an object-oriented application environment designed specifically for developing Mac OS X-only native applications. The Cocoa frameworks include a complete set of classes, and for developers starting new Mac OS X-only projects, Cocoa provides the fastest way to full-featured, extensible, and maintainable applications. You can bring applications from UNIX and other platforms to Mac OS X quickly by using Cocoa to build state-of-the-art Aqua user interfaces while retaining most existing core code.

GNUstep is the Free Software Foundation's effort to implement Apple, Inc's (formerly NeXT Software, Inc.'s) Cocoa APIs (formerly the OpenStep Standard).

All our applications are 100% code-compatible between Cocoa (MacOSX) and GNUstep (Sun Solaris, Linux,) and are deployed likewise on Macs and other Unix systems by making use of GNUstep. The following picture shows an app that accesses a 50GB database, accessed by 50 users simultaneously with the database being replicated to three different locations.

Though we mainly develop on Macs even our dev tools (clones of the original OPENSTEP dev tools) are available in both worlds. The following pictures shows a GNUstep/Etoile desktop running our ProjectBuilder.app clone and ApplicationBuilder.app. ApplicationBuilder.app allows to efficiently develop native and web-based database applications.