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.
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.
The MiscKit Frameworks - - A free collection of Objective-C objects useful to Rhapsody, OPENSTEP, GNUstep, and NeXTSTEP programmers.
OpenStep Specification - - A copy of NeXT Computer's OpenStep Specification. This is the specification that both Cocoa and GNUstep are based on.