I've already posted about this issue in the past but now I'm going to make some new names:

• Ineedfile(dot)com is often among the top results of many queries but isn't helpful at all. As far as I understood it's some kind of rapidshare search engine but it shows up when I don't type "rapidshare" and doesn't contain any information regarding my query.
• Filestube, a similar site in purpose, does the same. It even takes your queries and turns them into something completely unrelated.  
• Torrentreactor does the same
• Other sites that do it: Fileguru(dot)com, Fulldls(dot)com, Torrents(dot)net, Torrenthound(dot)com, Vertor(dot)com. I don't think you should remove torrent trackers from results, I think you should only keep the ones that are somewhat trustworthy and have large communities.
  
• Various pseudo ebook sites do the same. You look for information regarding a book and they pop up, without offering you anything. I can't name a single one but a lot of sites that have "ebook" and "pdf" in their URLs do that and it's annoying.
  
• Additionally, I would get rid of Craiglist advertisements in the results because they expire.
• Yahoo answers is really bad at giving answers. The questions are often left unsolved and the answers sometimes are wrong and badly written. Sometimes they are even unrelated to the queries. I would give it less priority, personally.
  

I got those results by typing queries like "UNIX book", very simple yet very crucial. I hope that's helpful.