Cocotron (without "a") and GNUStep are intended as source-level compatibility layers. You replace Apple's libraries with them at compile time, and obtain a native binary or the target platform.

This project is akin to Wine. It allows to run OS X binaries on Linux, without recompiling.

The Cocotron blog hasn't been updated in a while, but the project is still alive: http://code.google.com/p/cocotron/source/list

hm nope. Looks like Darling is meant to provide a binary compatible interface (Darling is similar to wine except that it targets OSX instead of win32/64) whereas Cocoatron provides a source code level of compatibility (you have to build your app with Cocoatron libs/APIs).