You can use GNUstep Renaissance on top of the following
- GNUstep, which runs on GNU(Linux) and most unix systems,
and (experimentally) on Microsoft Windows.
- Apple Cocoa, which runs on Apple OSX.
Applications written using the OpenStep API for their core code,
Renaissance for their user interfaces, and gnustep-make for their
makefiles will build and run with almost no changes in source code
under GNUstep and Apple OSX.