It's been moved into one of the add-on packages for Xcode, and is now part of the **Graphics Tools for Xcode** package.

Apparently this is (at least partly?) because this is no longer the 'officially recommended' way to generate (high resolution) icons. See the [OS X Human Interface Guidelines][1] and the [High Resolution Guidelines][2] for more info.

I believe Icon Composer doesn't support the highest resolution icons required these days (1024x1024), and you're encouraged to use `iconutil` instead, which does, and allows you to convert between ICNS and "iconsets" (effectively just a folder containing a collection of PNG files of different resolutions).

-----

If you still want it, you can get it as follows:

In Xcode, go to the **Xcode** > **Open Developer Tool** > **More Developer tools...**.

You'll need to log in (or register) with your (free) Apple Developer account, and then you'll be shown a list of available packages for Xcode, including the Graphics Tools for Xcode. Grab the latest one and install.

This DMG of Graphics Tools contains:

 - CI Filter Browser
 - **Icon Composer**
 - OpenGL Driver Monitor
 - OpenGL Profiler
 - OpenGL Shader Builder
 - Pixie
 - Quartz Composer
 - Quartz Composer Visualizer
 - Quartz Debug

  [1]: http://developer.apple.com/library/mac/#documentation/UserExperience/Conceptual/AppleHIGuidelines/IconsImages/IconsImages.html#//apple_ref/doc/uid/20000967-SW1
  [2]: https://developer.apple.com/library/mac/#documentation/GraphicsAnimation/Conceptual/HighResolutionOSX/Optimizing/Optimizing.html#//apple_ref/doc/uid/TP40012302-CH7-SW2