It seems that Cocoa seems to be the main platform for Objective-C. GCC (which Xcode uses) supports Objective-C so it must be available on a wide range of platforms.
Are there any notable cross-platform projects that use Objective-C but not Cocoa (or its open source cousin GNUStep)? Is it really used outside the Apple ecosystem?
Objective-C has also been popular in the scientific and financial services communities. There are still many Objective-C based applications deployed in banking, mostly on the trading analysis front. A friend works on a nearly million line of code Objective-C based analysis and trading engine for which they have written their own class hierarchy from scratch.
At one point, one of the more popular Linux window managers was written in Objective-C. That was a few years ago and may no longer be the case.
The LLVM compiler also compiles Objective-C, including having full support for Blocks. It is quite portable, too.
There is also David Stes's Portable Object Compiler. It compiles Objective-C to C and uses a class hierarchy that is based quite directly upon the original ICPak class hierarchy from StepStone.