Here's one of my latest referrals. Turnitin.com is an anti-plagiarism site that has the entire web as its database. You can submit a paper and then see if anything in its database is similar, using algorithms to see if the same keywords are there but rearranged. I've used it in the past, but I've found Google to be just as effective in catching plagiarism. This is a turnitin.com cache of one of my posts, with the plagiarized sentences highlighted in green. So someone wrote a paper, used those lines, and submitted it to someone who then checked it on turnitin.com.
This isn't the first time my blog has been plagiarized. One time it was by one of my own students. That wasn't very bright, was it?