I spotted Stephen Downes Referral System via www.davidwatson.org.

Very cool little script. I have added it to my templates here on high context. It will display the referrers to each individual page. So if you link to anything on this site your page will be listed on the sidebar as people follow the link.

One small bug I’ve discovered with Mozilla is that a local referral within the same site creates an unexpected value for document.location in the javascript code. No problems in IE so it must just be Mozilla.