"Download" pop-up button appears

I'm getting a weird "Download" button popping up on various websites, Facebook, etc., sometimes when I roll over things. It seems almost random...a gray button with the image of an arrow pointing down into a tray with the word 'download'. When I click it the button changes to "Please make the video is being played or it is supported". It just started popping up recently. Only seems to happen with Safari 5.0.3. It's annoying and there's nothing there to download anyway.