YouTube Fast Search is a cool web application that uses YouTube's API to create a way to both play videos and search for new videos at the same time. You can build a playlist from search results and add new videos to the playlist without interrupting it. The interface is similar to the one from MSN Video, but you can also edit the playlist and enlarge the video. Too bad that there's no option to save the playlist to your YouTube account or to import an existing playlist.
YouTube has the option to add videos from search results to a Quicklist and then play all the videos from the list, but you need to switch from the view mode to the search mode.