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GNUStep allows you to write Cocoa apps on OS X and simply recompile them for Linux. The Linux version will look old and gray, but that's actually kinda what "native" looks like on X11 systems.
I'm not sure GNUstep is entirely Cocoa compliant, but you probably could compile most GNUstep apps under Cocoa. Then again, hardly anyone uses GNUstep, so most users would avoid an application that depends on yet another library. Just installing GNUstep's Minesweeper clone takes almost 50 MB because of that, and it behaves like a true alien under KDE, with its own desktop menus and dock icons.
So yeah, it's native and ugly under Linux, but also brings along with it parts of the Nextstep desktop. It might be interesting to try it under Windowmaker, but who uses that nowadays?
I do. I kinda oscillate between Window Maker, StumpWM, and GNOME, depending on the system specs and my mood.
With Étoilé GNUStep looks nice...
Another good option is wxWidgets, it allows a real nice native touch after more than 10 years of improvements.
I'm not sure of the progress of these efforts, but a GNOME theme was working but unfinished on GNUstep somewhat recently:
There are a couple other posts about this on that blog, which is written by the maintainer of GNUstep, Gregory John Casamento.
More on GNUstep themes:
Your best bet for current information is the mailing lists:
http://wiki.gnustep.org/index.php/Get_Help Edited 2012-01-24 05:57 UTC