Bundles are a concept in Mac OS X whereby a specially named folder becomes what appears to casual users to simply be a file that can be copied as usual and often launched by double-clicking on it.
Other operating systems have file formats that are little more than containers for other files and in doing so keep those interesting resources out of your reach. (Okay, we have DMG but that’s more of a transportation mechanism like ZIP, TAR etc.)
To see the contents of one a bundle select it and just choose Show Package Contents from Finder’s context menu (hit the Task button in the toolbar, Ctrl-click or two-finger click).
The ones I find most interesting or useful are…
All sorts of great resources can be found in here including icons, templates, sounds and the NIB’s for the user interface. Great for learning but consider the license before you take resources for your own application.
Garage Band project/template (.band, .wand)
Audio files of the instruments and output can be found inside.
Safari Download (.download)
An absolute gem if you are downloading audio or video files and want to watch or listen to what has downloaded so far.
Slideshow Screen Saver (.slideSaver)
Various images used in the slideshow.
iDVD theme (.theme)
Various images in tiff format and Quartz Composer compositions you could try using as screen-savers.
Time Machine (.inProgress)
If you have a partial backup with Time Machine and need a file it my have backed up this could be a life-saver.
Your original and modified photos from iPhoto and the XML data behind them.
iMovie project (.rcProject)
Contains a QuickTime movie of all the still images used in the project.
There are others to be found scattered across your file-system too:
|.xcDataModel||Core Data model|
You might also want to check out this guide to examining bundles.