Grammarly

Note: Sometimes it's too aggressive though will label 4 words in a row that are the same elsewhere on the internet as plagiarism, but it is definatly good at checking....if not too good.

I've generally found that copy-and-pasting a couple of suspicious-looking sentences into google will locate a source (or sources), and then a quick visual comparison of the student's essay with the online text(s) will uncover enough to sustain a charge of plagiarism. I've also had some teaching assistants who were quite good at this and quite zealous in tracking down cheaters because they felt that their own degrees could be devalued by widespread cheating at their university.

In addition to the above answers here are some links comparing free tools available:

https://elearningindustry.com/top-10-free-plagiarism-detection-tools-for-teachers

https://www.quora.com/Which-are-the-best-plagiarism-checkers-for-online-content

https://yourstory.com/read/f65e68ea8c-top-10-free-online-plagiarism-checkers-tools-in-2017