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.