ActiveDeveloper: Interactive runtime Objective-C IDE for Windows, Mac OS X, OpenStep NIS.
Cocoa: Apple's Objective-C based programming environment for Mac OS X.
gnome-objc: CVS home of Objective-C bindings for Gtk+, GNOME.
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.
OpenStep Specification: A copy of NeXT Computer's OpenStep Specification. This is the specification that both Cocoa and GNUstep are based on.
Portable Object Compiler: A preprocessor that will convert Objective-C into regular C.
The MiscKit Frameworks: A free collection of Objective-C objects useful to Rhapsody, OPENSTEP, GNUstep, and NeXTSTEP programmers.