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Well, there's GNUstep... I'd be interested in knowing if anyone has used it recently and how complete it is.

It is dead, last time I checked they were still trying to reach parity with Panther.

Not dead at all. Microsoft has been using it to bring Obj-C to Windows. https://github.com/Microsoft/WinObjC

I thought WinObjC died alongside the Windows Phone efforts, and I do Windows development.