At Uni, they use a product called TurnItIn (
http://turnitin.com/) to check for plagiarism. Ironically, on assignment I tried it on returned an 80% or thereabouts likelihood of plagiarism because: (a) I was analysing a well-known ancient text; and (b) I cited my references. Every reference to the text and other discussions of it was reported as a 'hit'. Not much good at differentiating between legitimate referencing and plagiarism, it seems. Still, it might be useful for detecting cheating where a paper has been copied and both share a lot of phrases in common. I doubt it would be much good if one simply took someone else's ideas and expressed them differently.