Be very careful with free plagiarism tools. I have read that some of them steal your articles and before you know it they are published on the web. Among the free ones that I trust is Plagium. When you have hundreds of articles to check then this is an economical alternative. Among the paid tools, there are none as good as copyscape.
here is another plagiarism checker website which is exceptionally outstanding...you must try this one...http://www.academicplagiarism.com