Hello Michele, similarity checking is a pretty sophisticated technology if we are to produce high quality results. That is because not only you have to have a first class search engine (much like Google has and even better in some ways), but also, have an access to web index. It is possible to use offline cache which is significantly smaller than live web index, and still the engine quality matters. Free checks that are available online could provide mediocre quality at best due to simplistic search engine. And I am not mentioning other facts like integration with Canvas/SpeedGrader for automatic checks, search vs Open Access, privacy & security when working with such tools and many more factors such as usability, reliability, mobile devices support etc.. Overall, no free tool can offer such service, unless it finds a model how to compensate the development of technology and its continuous support (servers, fixes etc.). All this might sound like I am advocating for a paid tool at all costs - not at all. It's just a reality of these days, that free checkers simply offer poor experience for educational institutions and corporate use.
At Unplag we're working hard to create the best originality checking service for all educators. We have a huge roadmap of improvements and new features are coming by the end of this year and at the beginning of the next year (like Open Access search, Draft checks and others). Also, no free checker will offer you anything more advanced than an exact match. Here, at Unplag, we develop a unique technology to find synonymized matches as well as paraphrase and even more (soon we will be sharing this information with the public).
Feel free to email me at ivan@unplag.com and I will set up a live demo for you to show how everything works. No obligations. You can also have a free trial and run pilot project before you decide whether you want to move with the tool. Also, I'd like to mention that Unplag is an Alliance Partner of Canvas already.
