YOOPLAYER is a great solution to this problem. With this application, you can easily watch online videos on your desktop, whether they be from YouTube, Vimeo, or Viewster. This application will give preference to the window playing the video. Once it's set up, you can adjust the player's size or change other settings, like the subtitles or the resolution.
Using YOOPLAYER is really simple as long as you use its built-in browser to visit different sites and find the video you want to watch on your desktop. Then it will start to play automatically, and you can watch it while you do other things.