DOC Cop is a Plagiarism detection website. It can check PDF files as well as any version of Microsoft Word. DOC Cop is completely free to use and for the first time that I have seen of any of these sites, it does not keep the submissions. It deletes the document once it completes the submission. No submission is copied, retained, or passed on to be sold.
The idea behind DOC COP is really to compare papers over the same topic, not necessarily papers to other books, magazines, or websites. It is good if you have the resources in PDF form to check against, otherwise you are only able to check a students paper against another paper.
All the files must contain at least 20 words and no more than 100,000. All checks on web based submissions have a limit of 550 words. If you want to check a web submission you can do so, and it is powered by a Google Search. This would be good to check web pages that students created to see if they copied any material.
Sign up is as simple as providing an email and waiting for a Guest ID to be emailed to you. In the time it took to write this post I had not received a Guest ID though.