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.
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.
OS X Lion - When moving multiple files in Mac OS X Lion, an iOS-like bubble shows the number of files being dragged.
/via Hugo Ahlberg
Mac OS X Lion - Hold down the character key on your keyboard for a popup menu of the accented variants of said character. Use keyboard numbers for quick selection.
/via Jan van der Asdonk
Lion Safari - The scrollbar changes the color depending of the background of the current website. (dark or light)