I'm working on a MacRuby project in Xcode.  I upgraded Xcode from 4.0 to 4.0.1 and Interface Builder stopped working right for my project.  New outlets weren't showing up so I couldn't connect them to UI elements.  I eventually realized that in the Xcode upgrade the MacRuby templates got wiped out.  A reinstall of MacRuby got things working again.

Filesystem Hierarchy Standard for Mac OS X

I frequently refer to the Filesystem Hierarchy Standard when trying to figure out where to put files on Linux systems.

I recently started doing some Mac development and was looking for an equivalent document. Apple's File System Overview seems to cover similar ground.