Free Tools for Detecting Plagiarism

Google and Google Scholar: If a sentence strikes you as odd, put it in quotation marks and run a Google search on it. If the student cut and pasted the phrase, it will show up on Google. And as more books are uploaded onto Google Books, Google Scholar and Google Books will become increasingly powerful weapons against plagiarism.

The Plagiarism Checker: The Plagiarism Checker allows you to run a Google search on large blocks of text. This is easier than cutting and pasting sentence after sentence.
Articlechecker: Works the same as Plagiarism Checker, but gives you the option of checking against Yahoo as well as Google.

Plagium: Like The Plagiarism Checker, this site Googles text you submit. Unlike most other checkers, Plagium works in several languages.

PlagiarismDetect: A plagiarism detector that allows you to upload whole documents rather than cutting and pasting blocks of text. It's free, but you have to register.

Duplichecker: Another checker that plugs submitted text into search engines. Duplichecker's interface makes it easy to submit entire documents as well as excerpts.

SeeSources: Searches the Web for sources similar to the text you entered. You can scan both excerpts and whole documents.

DOC Cop: Doc Cop offers a few features more than the minimal Web-based detection services. For instance, you can check for collusion—that is, you can check the similarity between two papers. However, you do have to register.

WCopyFind: WCopyFind is a downloadable scanner that checks for similarities between two papers, but it can't search the Web.

Viper: The Anti-Plagiarism Scanner. Although it's free, Viper is software, so it's a bit more of a commitment than Web-based tools. However, it has some neat features, such as side-by-side comparisons of the submitted text with the potentially plagiarized one. Viper touts itself as the free alternative to TurnItIn.

SafeAssign/MyDropBox: This is free if you're already using a Blackboard Learning System. As students submit papers to Blackboard, SafeAssign checks their papers against its database of source material.

PAIRwise: PAIRwise (Paper Authorship Integrity Research) can compare documents to one another while searching the internet for similar documents. However, PAIRwise is intended for use on an institutional level—for departmental or college-wide servers.

Source: http://www.guidetoonlineschools.com/online-teaching/plagiarism#ixzz1Eav6tQQg