YouTube added a new feature that shows query suggestions in the search box. While you're typing characters, the search box scrolls down showing some suggestions to help users to find what they're looking for. YouTube Suggest is an auto-complete feature that functionality works like Google Suggest, so it borrows a lots of tricks from Google Suggest.
The new function doesn't works on YouTube's main site, so it's currently available on YouTube UK and works only over homepage.
Read also on Google OS.