Display Power Saver (Free) for Android
Unlike other apps which works only for YouTube and that too just by streaming audio, this doesn't mess with the actual content. For most part, I just turn this app on and hide the black display whenever I want to watch a part of the video clip. Also realized that it works with flash videos in chrome browser. Its great at work too, folks just look at a blank display and have no idea :-)
by D####:
Superb