nicovideoautoplay.user.js

// ==UserScript==
// @name           NicoVideoAutoPlay
// @namespace      http://d.hatena.ne.jp/javascripter/
// @include        http://www.nicovideo.jp/watch/*
// ==/UserScript==

location.href = "javascript:" + function () {
  var flvplayer = document.getElementById("flvplayer");
  (function play() {
    try {
      if (flvplayer.ext_getStatus() == "paused")
        flvplayer.ext_play(1);
      else
        setTimeout(play, 500);
    } catch(error) {
      setTimeout(play, 500);
    }
   })();
}.toSource() + "();";