OS X emulation layer for Linux
About a week ago, the author posted a report to his blog http://www.abclinuxu.cz/blog/doli/2013/7/darling-progress-re....Summary:  * It can run basic console programs like Midnight Commander, vim
    or bash, so compatibility of system API is rather good.
  * Dynamic loader is being refactored, because it can not unload
    dynamic libraries now. In the future there will be API that
    would allow loading OS X library into native code.
  * There is a FUSE module for read-only access to .dmg files, so
    that it is not necessary to unpack them.

GNUstep is a problem; it does not seem to be very alive. It does have three backends, but none of them works reliably. The long term (slightly crazy) plan is to implement the basic Cocoa classes over Qt.Right now the target application that should work is Angry Birds.