Plagtracker for FREEMIUM http://alternativeto.net/software/turnitin/?license=free UNPLAG FOR COMMERCIAL (based on alternativeto.net popularity) Unfortunately we lack a reliable & 100% OpenSource
ALSO CHECK THIS TEXT ADDED TO SIMILAR AND PREVIOUS/RECENTS SIMILAR ANSWERS ON RG, there you fill find a huge plethora of options. Being the best list of all the top 10 https://elearningindustry.com/top-10-free-plagiarism-detection-tools-for-teacher and the 26 TESTED EFFICACY http://plagiat.htw-berlin.de/software-en/2010-2/. My academic institution uses Turnitin so it must count as a reference, so check for the one used at your institution library!! they must or can grant you access if you're teacher...
(THIS my-text tries to summarize a revision to the six RG question) I haven't used plagiarism cheking software, but the provided answer are based on concise and popular users who insistently tried to persuade us NOT USING VIPER, because suspiciously they sells the provided docs for revision)