Cocotron is a developer SDK which implements a usable amount AppKit and Foundation for Windows and Foundation for Linux/BSD in Objective-C. You need to install cross-compilers and cross-compile the frameworks using Xcode on Mac OS X.
The Cocotron is an open source project which aims to implement a cross-platform Objective-C API similar to that described by Apple Inc.'s Cocoa documentation. This includes the AppKit, Foundation, Objective-C runtime and support APIs such as CoreGraphics and CoreFoundation.
The purpose of the project is to provide an easy to use cross-platform solution for Objective-C development. In particular, source code level compatibility with recent versions of Apple's frameworks.
The general goal is to provide complete support on any viable platform, the project is intended to be as portable as possible. However, most of the work at this time is focused on providing support for Microsoft Windows. In particular the NT based versions, 2000 up to 10.
The Cocotron proper is only source code to a runtime, without a compiler and sufficient tools this is relatively useless, so the project also maintains The Cocotron Developer Tools (CDT). The CDT is a collection of well known and popular open source tools, specifically gcc, binutils and MinGW, which are organized and patched to support the Cocotron project.
The CDT is intended to be used from within Apple Inc.'s Xcode as a cross-compiler, and Cocotron itself is an Xcode project. Since the majority of Objective-C developers are on Mac OS X using Xcode this allows one to develop and maintain a cross-platform project within one IDE.