Google Analytics has become the defacto standard for website tracking. Its incredibly easy to add to a site and provides nice pretty graphs . It is so awesome that even  Alexa uses it! Copy and pasted proof after the page break.

<!-- START GOOGLE ANALYTICS --><script type="text/javascript">  var gaJsHost = (("https:" == document.location.protocol) ? "https://ssl." : "http://www.");  document.write(unescape("%3Cscript src='" + gaJsHost + "google-analytics.com/ga.js' type='text/javascript'%3E%3C/script%3E"));</script><script type="text/javascript">  var pageTracker = _gat._getTracker("UA-2146411-1");  pageTracker._initData();  pageTracker._trackPageview();</script><script type="text/javascript">  function trackCustomEvent(customEvent) {    var eventname = "/custom-event/topsites/"+customEvent;    if (pageTracker != undefined) {      pageTracker._trackPageview(eventname);      return true;    }  }  function trackExternalLink(externalLink) {    return trackCustomEvent('external-link/'+externalLink.replace("http://", ""));  }

</script>

<!-- END GOOGLE ANALYTICS -->

<script type="text/javascript" language="JavaScript" src="http://atrk.alexametrics.com/atrk.js"></script><script type="text/javascript" language="JavaScript">_atrk_opts = { atrk_acct: "s3LE913x9k00WW", domain:"alexa.com", label:"topsites" }; atrk ();</script><noscript><img src="http://certify.alexametrics.com/atrk.gif?account=s3LE913x9k00WW&jsv=20080822" style="display:none" height="1" width="1"></noscript>
<script>  $('a.offsite').click( function() {    trackExternalLink(this.href);  });  $('a.social').click( function() {    trackCustomEvent( "social/" + this.name );  });  $('a.trackme').click( function() {    trackCustomEvent( "link/" + this.name );  });  $('button.trackme').click( function() {    trackCustomEvent('button/'+$(this).text());  });$('a.trackme-cross').click( function() {			      trackCustomEvent( "link/" + this.name );			      selectTab('#keywords');			    });

</script>