You can add it as a bookmarklet by dragging this link up to your bookmarks bar: slitscanner. Just hit the bookmarklet on any YouTube or Vimeo video page with an HTML5 player, and it will start drawing onto a canvas in the browser. You can modify the speed with the slider (the default value draws the entire video to the width of the browser). You can also download the code and modify it as you wish.
Sadly, due to crossdomain issues I don't know of an easy way to turn the slitscanned canvases into images other than taking a screenshot, so for now it will remain as more of a personal toy. I'm posting nice ones I generate to this tumblr and tagging everything with #slitscanner. If anyone wants to put up a proxy server to serve video with CORS enabled, let's talk.
There is also an incredible collection of slit-scan artwork put together by Golan Levin here.