Well, there is also the GNUStep framework that some people still develop for. I don't know how active that community is, however.
But for the most part Objective-C is now an Apple language, as illustrated by the fact that the "2.0" version of the language was launched as part of OSX 10.5 and seems to have been a purely in-house development effort at Apple.
Edit: Apple, as you probably know, has something called the Cocoa framework, which basically refers to the entire stack of libraries for Objective-C, including the NSObject common baseclass. Cocoa is a continuation of the NextStep library, which GNUStep is the open-source version of. So you'll find many of the NS-prefixed classes both places, but Apple has not held back in adding new classes or changing them in Cocoa, so interoperability at the GUI level is really only a theoretical possibility.
