nicovideoautoplay.user.js

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

// http://d.hatena.ne.jp/javascripter/20081003/1223033435

function evalInPage(fun) {
  location.href = "javascript:void (" + fun + ")()";
}
evalInPage(function play() {
    var flvplayer = $("flvplayer");
    try {
      if (flvplayer.ext_getStatus() == "paused")
        flvplayer.ext_play(1);
      else
        setTimeout(play, 500);
    } catch(e) {
        setTimeout(play, 500);
    }
});