If you write good copy and publish on the web, chances are that someone has copied/plagiarized it. While in reality, there is very little you can do to stop the unethical practice becauase of the effort involved in pursuing legal action, its always interesting to know who is stealing/plagiarizing your text.
Copyscape.com is an online free plagiarism checking service that does exactly this and offers various free and paid options to see who has used or adapted your content.
It works pretty well too. Just type in your URL and the service takes care of the rest.
Visit: http://www.copyscape.com/
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