Many of my Business Advice clients are concerned about rivals stealing their on-line content, so I was pleasantly surprised to come across the free Copyscape service whilst researching a replacement mobile phone on the Mobile Phones UK: Reviews & Best Buys website.
All you need to do is post in your website address and see where (and how much of it) is appearing elswhere on the Web. Needless to say Copyscape offer a premium service with no monthly limit and batch searching for a fee.
Copyscape recommend you put a warning notice on your website to help scare off any potential content theives.
Defend your site with a plagiarism warning banner!