Going through my analytics reports, specifically the list of referring sites. I always visit the referring site to see where they have put the link to me.
I am finding a number of sites that have supposedly referred visitors to me, but when I have a look there is no link to be seen!
Now, I reckon this is "log file spam" where a bot has visited my site with fake HTTP info making it look like a referral. So by me visiting the "referring site" the spammer (or more likely blackhat SEO) has got what they wanted .. a unique visit to the site that they can claim as being traffic they have generated.
I am thinking of contacting such websites to ask if they are happy paying someone for such underhand techniques.