I've used Floating Youtube Player, it only works with youtube but it's pretty much the only video app I use, oh I'm not ruted on this gs3 and its a free app...
its on the google play store, you pretty much download it and then go to youtube to whatever video you want and then go to the top right to share, the floating video option appears, you select it and it makes the video float on the screen.
It's awesome cause you can do almost anything else at the same time and you can also lock the phone and still keep playing the video (perfect for full album or concert vids).