With YouTube Fast Search you can create your playlists dynamically. At the same time you're creating your playlist(simply drag & drop from search result), you can drag & drop video you want to play into the block above. As you see on screenshot below, you can also pause, enlarge and slide the current playing video.
The cool feature is that you can create video wall. Clicking on VideoWall will launch you to another page where you can search and get results to create video wall.
Through scrolling down of the main page where you were creating your playlist, you will find two another features that you both can add video search to Google Toolbar and get the script. Clicking on Get this script will launch you to another site where you can get the script.
See instructions and examples here.