I noticed this answer on Super User this morning, obvious advertisement for some commercial plugin. Quite usual situation, happening frequently.
What is more surprising, though, is the amount of upvotes this user got for his other answer, advertising one as well. The answer was clearly off topic, but it got highly upvoted, very fast.
Following this track, we can see another user advertising the same product, once again with quite good reputation (for an advertiser, of course). His only answers are about this product, and the other one is where we reach a wonderful, magical place!
The place where everyone agrees that this product is wonderful, ideal, so great, and all with high votes! This question has obviously been cleaned up already, many of the users are deleted (we could probably get rid of their CW posts as well), but it shows really a "farm" of advertising users, and smarter than the usual.
For example, this one posted a valid question, to gather enough reputation to be able to vote. And same for others, finally. That way, they advertise, agree with each other, and upvote each other!
Edit: Even better, they use questions to criticize a competing product, and accept their own as a correct answer to that!
So if some moderators are motivated to make a bit of a spring cleaning, a Google search shows most of the posts to clean (I think only one of them is legitimate), and the users with it ;-)
We need to take down the whole gang!