Last year, a friend of mine who was doing adjunct teaching at Eastern Michigan University posted on Facebook that in a class of 30 students who had just turned in papers, she had realized that five of them (5!!!) had turned in papers that they found on the internet. At least some of them downloaded the same paper! All of them had B or better averages before they turned in their final papers...
In any case, plagiarism is a problem, whether you teach middle school, high school, or college, and the Electric Educator (John Sowash) has a post with some suggested tools for checking for plagiarism.
A free, simple, alternative that I have been using for several years is "The Plagiarism Checker" from dustball.com. Paste in a bunch of text from a suspect paper and The Plagiarism Checker will quickly perform a Google search of multiple portions of the submitted text."
Read the rest here.