Facebook like button that validates. Now outdated, see http://developers.facebook.com/docs/reference/plugins/like/

gistfile1.html

<script type="text/javascript">
  //<![CDATA[
  (function() {
      document.write('<fb:like width="200"></fb:like>');
      var s = document.createElement('SCRIPT'), s1 = document.getElementsByTagName('SCRIPT')[0];
      s.type = 'text/javascript';
      s.async = true;
      s.src = 'http://connect.facebook.net/en_US/all.js#xfbml=1';
      s1.parentNode.insertBefore(s, s1);
  })();
  //]]>
</script>