OSX - Excellent integration of the MacApp Store into the OS for unknown file types.
/via Pete Wilkinson
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
Google Mobile Weather - Background color changes according to the time of searching.
Mac OS X - When doing a calculation in Spotlight, Cmd+C copies the result without having to select it.
Chrome - Marks the position of cmd-F search matches on the scrollbar.
/via Saul Cozens
Apple - Menu drops down and bounces a little bit. Search bar expands intelligently once activated and shows different search results depending on what page you are on.
/Thanks to Renato for being the first to submit.
Rdio - Interesting search dropdown with grouped results
/via Chris Bedoya