Amazon - Drop down submenus detect/calculate the direction of the cursor to determine whether to keep displaying a submenu or move on
YouTube - When typing in the search box, the autocomplete dropdown size will be limited so it doesn’t cover the video.
Facebook - When “new family member” is added, Facebook automatically recognizes the chosen family member’s gender and adjusts the list of possible family relatives in the list box accordingly.
Pages - Automatically adds smaller heading styles once you’ve used the smallest displayed style (e.g. “heading 3” only appears after you have used “heading 2” in your document).
Google Docs - Lets you choose your table layout in a very visual way.
Amazon - Changes the categories drop down on the left when you tab down to an autocomplete that is related on the right.
Rdio - Interesting search dropdown with grouped results
/via Chris Bedoya