PaperRater.com

The best time to check your paper for errors is before you turn it in.  

PaperRater.com uses artificial intelligence to detect grammar errors, punctuation issues, and spelling mistakes as well as give you tips to help make you a better writer. You can use the free features without having to signup, login, or download anything.

PaperRater.com also offers FREE plagiarism detection.  Simply upload or copy and paste your paper into the box.  Select your education level, 1st grade through Doctorate.   Enable Plagiarism detection and select continue.  

The free version does have some limitations: 

Maximum 5 pages per paper (longer papers can be broken up into separate submissions)

Maximum 50 submissions per month

Maximum 10 plagiarism checks per month

Despite these limitations, it is a great tool for writers of all ages.  Try it out for yourself the next time you have a paper to write.