GNUstep

A GNU project to write a completely OpenStep compatible programming environment based on Objective-C, with intentions to also maintain Mac OS X compatibility.
http://www.gnustep.org/

ActiveDeveloper

Interactive runtime Objective-C IDE for Windows, Mac OS X, OpenStep NIS.
http://www.interactive-technology.com/

Cocoa

Apple's Objective-C based programming environment for Mac OS X.
http://developer.apple.com/cocoa/index.html

Portable Object Compiler

A preprocessor that will convert Objective-C into regular C.
http://users.pandora.be/stes/compiler.html

The MiscKit Frameworks

A free collection of Objective-C objects useful to Rhapsody, OPENSTEP, GNUstep, and NeXTSTEP programmers.
http://www.misckit.com/

OpenStep Specification

A copy of NeXT Computer's OpenStep Specification. This is the specification that both Cocoa and GNUstep are based on.
http://www.gnustep.org/resources/OpenStepSpec/OpenStepSpec.html

gnome-objc

CVS home of Objective-C bindings for Gtk+, GNOME.
http://cvs.gnome.org/bonsai/rview.cgi?cvsroot=/cvs/gnome&dir=gnome-objc