Search engines and SEO spam
The funny thing is that if the people who worked on spam at Google were free to talk about it, I'm sure it would become evident that they know more about spam and anti-spam efforts than anybody else in existence.  It's a ridiculously hard problem, especially when people are targeting you directly.  But they aren't free to talk about it, because if they did it would just give more assistance to the spammers, and make the problem worse.I'm not saying that curated search results for particular verticals is a terrible idea (though I'm sure like anything the devil is in the details), but on the whole Google search is very, very good considering the constant assault they are under from spammers (which most other search engines are not, at least directly).