I have seen several people discussing plagiarism and wondering about free plagiarism checkers. There is a site called Small SEO Tools that has a number of tools available for writers and website makers. I have not used any of the available tools except for the plagiarism checker. However, you may want to browse the site, they may have other things that will help you as well.
Small SEO Tools: http://smallseotools.com
The Plagiarism Checker: http://smallseotools.com/plagiarism-checker
Hope this helps!
Image Credit: I created this image from a screenshot. http://smallseotools.com