Blacks out / reveals the window. Useful as a bookmark (it toggles) — copy the minified version and stick `javascript:` in front of it.

blackout.js

(function () {
  var cover = document.getElementById('__cover');
  if(cover) {
    document.body.removeChild(cover);
  }
  else {
    cover = document.createElement('div');
    cover.id = '__cover';
    cover.setAttribute('style', 'background:#000;position:fixed;top:0;left:0;width:100%;height:100%;z-index:999999');
    document.body.appendChild(cover);
  }
}());

blackout.min.js

(function(){var e=document.getElementById("__cover");if(e){document.body.removeChild(e)}else{e=document.createElement("div");e.id="__cover";e.setAttribute("style","background:#000;position:fixed;top:0;left:0;width:100%;height:100%;z-index:999999");document.body.appendChild(e)}})();