I wonder if there is any RAD tool for Mac OS X. Something like Delphi for Windows.
Which languages are you interested in?
Certainly check out Cocoa, XCode and Interface Builder.
An actual Delphi for Windows that will allow you to cross compile your applications to Macintosh has been promised by Embarcadero (the current owners of Delphi).
The IDE will be a Windows product but the binaries it produces will run on a Mac.
It was due this year but wasn't ready for the September release of the latest Delphi. There are likely to be preview releases for existing Delphi customers in a few months from now but the actual product probably won't be released until around September 2011.
Lazarus is an IDE for Free Pascal that works on Mac OS X (among other platforms). Obligatory screenshot here.
Note: Lazarus supports two widget sets (Carbon and QT), with a third in Alpha (COCOA), most screenshots will be of the Carbon port nowadays.
Delphi Prism XE is an add-on for Visual Studio that supports both .NET as Mono. This includes the MONO version used by the Mac. It's not the ideal solution and I haven't been able to check if it really works, since I don't have a Mac. But I have Prism and now my .NET environment allows me to write Delphi applications for the Mac. Again, too bad I don't have a Mac, else I would have been able to tell you how good that is!. It appears as if the Delphi Prism IDE can also be installed as a stand-alone compiler with it's own IDE on a Mac. Again, I haven't tested this. I just know it has this option, since I saw it appear during installation.
As Frank Shearar already said, Lazarus is a Pascal IDE which would be a better solution for the Mac, right now.