AirDrop - The icon changes from a package with a parachute to a radar when selected.
Mac App Store - While downloading a new app, the icon transparency changes according to the progress of the download.
/via Florent Gosselin
OS X - The print queue icon matches the actual make and model of the printer/copier you are printing to.
VisualHub - Installation window: Put app inside the Applications folder, put User’s Guide inside your brain.
OSX - The little red close button displays a darker circle inside when your document has unsaved changes.
Chrome (Dev Channel): Displays “Hold ⌘Q to Quit” instead of immediately quitting to make sure you didn’t intent to press ⌘W.