Referrer Spam

A quote from SpywareInfo : Referrer Spam:

Bloggers quite often will display a link to the most frequent or most recent web site found in their referer logs using scripts. Some will even put those links right on the front page of their site in a sidebar area.

Unscrupulous web site owners are spamming the log files in order to have their web sites listed on those referer links. This creates an artificial boost in that site’s popularity among those search engines that measure the number of links to a site. It also generates traffic when curious visitors of a victim site clicks the links displayed in the referer listing.

Simply put, these people are running advertisements on your web site and using it to boost their search engine rankings. They do this without your knowledge, without your permission, and without compensating you in any way for the use of your network.