How to log the referring site

My web startup was deployed in June... It took me until this week to realize I should be recording the referring site for every signup. And no... Google Analytics is not a good solution for this.

The client side code goes something like this:

    <form>
     Email
     <input onfocus="javascript:document.getElementById('referringSite').value=document.referrer"
         name="email" />
     <!-- or with jquery: -->
     <!-- <input onfocus="javascript:$('#referringSite').val(document.referrer);" name="email" />-->
     Choose Password
     <input type="password" name="password" value="" />
     <input type="hidden" name="referringSite" value="" id="referringSite" />
     <input name="submit" type="submit" value="Sign up!" />
 </form>

So now, instead of knowing where I get the most hits from, I know where I get the most account creations from, which is what really matters.

Anyone know of any other super valuable info to record?