For any site owners serious about search engine optimisation, playing a safe & honest game with the search engines is essential - you don't want to be penalised for copying other people's content. Similarly you don't want other websites referencing your content verbatim, without an appropriate link back to you as the author.
Copyscape is billed as the world's leading plagiarism checker.
Search for copies of your web pages across the World Wide Web using their easy-to-use checker. It's also great to help you keep track of your online PR activity.
Read more and have a go at http://www.copyscape.com/
Updated March 2010