Take a look at Ryan Naraine's blog at CNet. Microsoft just published some interesting stats on Search Engine spam. This is one of my personal pet peeves. There are some very interesting results.
These same sites are being hit almost constantly for actual email spam as well. They send you to one of these URL's through an email, it auto-clicks on an ad for a product for you. You look like a unique hit to the search engine and they pay up for an ad click to these fraudsters.