Which free online academic plagiarism checker is closest to Turnitin?

I have recently (2 year ago) done a fairly thorough search for good, free alternatives to TurnItIn. My motivation was that my school did not use TurnItIn but that does not stop me from caring about my students' work. In the end, I found all of them (and there are many) are simply terrible in comparison to TurnItIn.

I've been using TurnItIn for 2 years now and I find it quite solid (of course there are things I would change but the basic functionality works well). The system has flaws but as I see, they maintain it and when a vulnerability is found (allowing a student to game the system), they generally fix it.

Add to this the fact that most often (my) students copy from previous students, searching freely available online sources will not do a proper job.

TurnItIn has a bit of a natural monopoly so it seems unlikely that any free alternative will ever gain much market share, meaning they will not have the papers to check leading to a vicious circle.

I see you are a high school student which makes me wonder why you want to know. Without knowing your true intention, I'm afraid my answer will be less useful to you.

If your purpose is to recommend a system for your school, they need TurnItIn. If your purpose is to check your own papers, you should use TurnItIn's student version (I know, it's not free).

I never tried any software thoroughly enough. If you google your question, you can find a lot of solutions, and it's just a matter of contacting the vendors and benchmark the different options.

The question you must ask is: what are the sources against which the system makes a comparison? If you only look for free sources, your possibility of finding similarities are limited. If your system matches also the high amount of academic literature hosted by publishers, the odds to find a match are higher. But since the highest amount of academic literature is accessible only via paid subscription, your anti-plagiarism system has to make some sort of agreement with the publishers.

So bottom line is: the best software is the one which searches through the highest number of resources. If a product is only limited to what is freely available on the web, it is very short-breathed.