nicolink to thumbnail.

(function(){
  var i,m,c={},f,d=document,p,a,an=document.getElementsByTagName('a');
  for(i=0;i<an.length;i++){
    a=an[i],p=a.parentNode;
    if(m=/^http:\/\/www\.nicovideo\.jp\/watch\/(\w+\d+)/.exec(a.href)){
      if(!c[m[1]){
        c[m[1]]=1;
        f=d.createElement('p');
        f.innerHTML='<iframe width="400" height="200" style="border:gray solid 1px;" src="http://www.nicovideo.jp/thumb?v='+m[1]+'"></iframe>';
        p.insertBefore(d.createElement('br'),p.insertBefore(f,a.nextSibling));
      }
    }
  }
})()