It's not uncommon that popular sites are often plagiarized word for word. It's probably happened to you, as much as it has to us. You might find tools like CopyScape offer some useful automated tracking but it's limited in scope. If you're looking for more, consider Tynt.com's Tracer (Beta).
Tracer is a simple one-line Javascript that you can add to your site or blog to monitor the content from your site which users find most engaging. This means, whenever users highlight or copy-paste any text or pictures from your site, Tracer tracks it. What's more, when users paste the copied content in an email, blog or another web page, Tracer automatically adds an attribution link back to your site, potentially increasing your site's traffic and improving your search engine rank.
What does Tracer tell you? The Dashboard gives you a tag cloud of the most copied words, a list of the most copied images and pages, as well as high-level stats for page views, copies, etc. over different time spans.
