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
Mac OS X - Pressing Alt ⌥ and clicking the volume icon lets you select input and output device.
OSX - Excellent integration of the MacApp Store into the OS for unknown file types.
/via Pete Wilkinson
Mac App Store - The progress bar of installing an app from the Mac App Store changes its dimension separate from the icon, when magnified in the dock.
Skype 5.0 for Mac - Displays an arrow next to a person’s mood message. When clicking on the arrow, all links will be opened at the same time.
Twitter for mac - After typing the first few letters of a word, pressing the ESC key will show a list of words that starts with those letters.
Edit: This is a native feature of OSX and works in some other apps as well.
Mac OS X - When searching in the help menu of an app, the menu relevant to the result is pulled down with a blue moving arrow pointing at the menu item.
/via Stephen Litt Belch