We've been inspired by YouTube while implementing this feature into MiwoVideos. You can quickly create playlists and add/remove videos to/from them from the "Add To" tab of any video page, just within a couple of clicks. All is Ajax based.
You can also create a menu to show the available playlists and let others play them.
The playlist player is also inspired by YouTube with video thumbnails & titles and previous/next control buttons.