YouPlayer is a YouTube video player with playlist similar to the one in Winamp. You may add/drag a YouTube link into a playlist window. When you double-click on it, it will be shown in the video window. You may drag a link to a browser window or right-click it and open in a new tab. You may also add/drag other links, thus the playlist will work as a bookmarker.
Download: YouPlayer 0.9.2