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

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

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

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

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

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

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/