Which is the free plagiarism software available online?

As for most cases, free plagiarism checking software have their on limitations. The limitations may be of the size of the file, the content and extent of report , the total no of checked files etc. Moreover they may have the issues of interference and data extraction and ads. Most free software allow only quick search and not deep search.
But if the plagiarism check is needed just off and on and not frequently or regularly, then the free versions can be used. Some of them need sign up. Many are available for online check straight.
I had used some of the following free plagiarism checking software at times in the past. My need was just ad hoc and for small contents once in a while. So those were just sufficient.

1.Grammarly Plagiarism checker
2.Duplichecker
3.Plagiarism checker Free
4.Papers Owl plagiarism checker
5. Plagium
6.Viper plagiarism checker
7.Pape rater
8.Plagscan
9.Quetext
10.PlagTracker
The above list is not based on the order of any parameter of utility or performance or experience, but listed at random.
You may try these, see the pros and cons and then choose the one(s) that suit best for you.

There is an article on this subject published on ISC. A beautiful article which gives the 5 best Plagiarism checkers. I am giving below the link for that article. I advise the author to go through that article for full details.
Top 5 Plagiarism Checkers for Students to Highlight Plagiarism in Thesis
As mentioned in this article the top 3 are very reliable.

There are several free plagiarism checker software available online, some of them are:

* Grammarly Plagiarism Checker: Grammarly Plagiarism Checker is a powerful tool that can detect plagiarism in your work and help you avoid accidental plagiarism. It checks your text against over 16 billion web pages, as well as its own database of academic papers, and highlights any matching text.

* Quetext: Quetext is a free plagiarism checker that can scan your text for plagiarism and provide an instant report. It also includes a feature that allows you to check for plagiarism across multiple documents at once.

* Small SEO Tools: Small SEO Tools offers a free plagiarism checker that scans your text for plagiarism and provides a detailed report. It compares your text to billions of web pages and documents, and can detect plagiarism from other sources.

* PlagScan: PlagScan is a plagiarism checker that can scan your text for plagiarism and provide a detailed report. It compares your text to billions of web pages and documents, and can detect plagiarism from other sources.

* DupliChecker: DupliChecker is a free plagiarism checker that can scan your text for plagiarism and provide a detailed report. It compares your text to billions of web pages and documents, and can detect plagiarism from other sources.

Keep in mind, these are just a few examples of plagiarism checker software that are available online. There are many other options available as well, so it's worth taking some time to research and find the one that best suits your needs.