Copyscape – Search for Copies of Your Page on the Web.

Googles standard response is to filter out duplicate pages (supplementary results), and only show one page with a given set of content in its search results. If someone copied your content, Google might wrongly label yours as duplicate contents. Google does penalize sites with large amounts of duplicate contents. Therefore, we need to defend our rights online, Copyscape is the leading provider of services that protect your content against online plagiarism and theft.
The free Copyscape service makes it easy to find copies of your content on the Web. Simply type in the address of your web page, and Copyscape does the rest. Copyscape finds sites that have copied your content without permission, as well as those that have quoted you. Act now!

Requirements: No Requirements
Demo: http://www.copyscape.com/
License: License Free