Persian City Directory: Computers: Programming: Languages: Objective-C: Compilers and Environments:

Web Directory - Computers: Programming: Languages: Objective-C: Compilers and Environments

See also:

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.
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.