I have 113 playlists in YouTube and I hate to scroll around to find the list I want to save a video into. Unfortunately this list is ordered by last usage date so it is not easy to find a playlist which was not used recently in a 4-5-lines-tall list.

When you press Save button on the video player page of YouTube (or at any subpage of Youtube where this same dialog is used, for example at Watch later -> Save to playlist) this script injects a search field into the save dialog. When you start typing only the playlists will be shown which contain the text you entered into the search field.