When NeXT shared their compiler patches and runtime (after some GPL-prodding), Objective-C became part of gcc and available on pretty much any supported platform, especially after the GNU-runtime delivered a portable messenger ( objc_msg_lookup()() vs. objc_msgSend() ).
That's the language itself, but you also need to consider the frameworks implemented in it.
NeXTSTep itself was, of course, ported to Intel, HP-Precision architecture and SPARC. Compiling for other architectures was as simple as checking a box in Project Builder. Later there were OPENSTEP-compatible implementations on Windows and Solaris.
The grand-daddy of the open-source implementations is, of course, GNUStep, though there are a bunch more that have been around for a while such as libFoundaton and mgstep. My current favorite is Cocotron, it allows cross-compilation for Windows and Linux right out of Xcode.