Mac

Depends on the language or platform. If you want something that can potentially go into the mac app store, then I would say XCode and Objective-C is your friend. Though I believe something like mac ruby will work for that as well.

I have mess around with C++ using XCode, which i managed to get a very basic side scroller working just fine. For when I would toy with Java, Eclipse does the job for me.

Though I recently started porting a sidescroller (same one i had in C++) from the Java version I had, into JRuby.

What tools do Mac desktop game developers usually use?

I agree with agm_ultimatetex; this largely depends on the the language and the project.
What kind of tools? Text editors? IDEs? Static analysis tools? Graphics manipulation/illustration? 3D modeling? Audio editing?

Here's what I use most of the time on OSX:

Java: Eclipse
All other code/text: vim
Graphics: GIMP, Inkscape
Static analysis:
Java: FindBugs, Checkstyle, Emma
Python: pep8, pylint, pyflakes

There's nothing OSX-specific about these tools; any reasonably civilized unix distro can run these tools.

• iOS SDK (Xcode 4.2)
  
• Android SDK (v3.0 and v2.3.3, with Eclipse)
  
• Corona SDK
  
• Titanium Studio from Appcelerator
  
• GameSalad
  
• Mono, MonoDevelop, MonoTouch
  
• Phonegap
  
• JetBrains AppCode
  
  As has already been pointed out XCode is pretty essential since it installs quite a bit of things that other development tools depend on.
  I also find Macports and VMWare to be quite useful.

Given that Unity was born there, i'm rather shocked it wasn't mentioned yet.