iTunes - When adding a free item to a Wish List, an alert reminds you that you can download it right away.
Mac OS X Mountain Lion Spotlight - Finds Calendar & Contacts even when you type in their old names (iCal & Address Book) from previous versions of Mac OS X.
OS X - Finder prevents you from accidentally modifying the extension when renaming a file, but only for known file types.
Mail - When selecting multiple messages the number of envelopes increases from a couple to many depending on the amount selected.
OSX Lion - Lets you number your files naturally and shows them in numeric order.
Previously you needed to number them with an appropriate number of preceding zeroes like f.e.: screen 01, screen 02, screen 03.
OS X Lion - The cursor reflects the fact that a window cannot be resized in a certain direction.
OSX - When unplugging your headphones from a Mac, it automatically reverts to the most recent volume settings of the new output device.
Mac OSX - The world map displayed within the Data & Time system preference panel changes with the seasons.
/via John Blackburn