Designers and UI tweakers alike should get a kick out of this one: Launch Preview app, and then drag any application into the Preview Dock icon to view and explore all of that applications graphical resource files.
Inside you’ll often find extensive amounts of graphics, icons, interface elements, screenshots, assorted images, and tons of other interesting stuff. In fact, sorting through the graphic elements of certain apps is actually how a lot of people were finding the 2x images that first gave rise to the retina Mac rumors some years ago.
Some applications will only show you icon files under a .icns dropdown, but generally the larger the app the more GUI resources will be included. Particularly fun apps to explore are Apple’s creations, which are usually packed with high quality images. Or if you want to see an epic load of system resources including the high quality Mac hardware icons, drag the entire /System/CoreServices/ folder into Preview to load up about 6000 graphic elements that form the user experience of Mac OS X itself.
This is kind of like browsing through picture resources with ResEdit back in the days of Classic Mac OS. Let us know if you find anything particularly worthwhile.