Grammarly is an excellent online tool to proofread writing, check for grammatical errors, and look for plagiarism within writing. As an educator, I would use Grammarly to analyze and check student writing, but would also offer it to students to improve on their own writing.
Plagiarisma,Net is a free plagiarism checker service available online that I just found. Try it out, It’s simple to use! Here is also an excellent Edutopia article by Kim Bochicchio, a Pennisulviannia teacher on her dealings with Plagiarism in her classroom.