Duplicate Content
Content theft and duplicate content is a huge problem for Google as of now. It is easy for people to lift your source code and post it onto their own site. This will however lead to some sort of duplicate penalty being applied to the site, depending on Google’s discretion. If your site's position in the search engines has recently plummeted then duplicate content might be the cause or if a competitor is found for text or an article you wrote then you may also have to face an issue.
It is an unpleasant task to check for duplicate content but there are efficient tools available to check duplicate content, such as www.copyscape.com. Just by entering the URL of the page you want to check, Copyscape will return a list of pages in the Google index that contain text also present on your site, for more detail you can also subscribe to their Copy sentry service.
If you're tempted to modify the content then you will need to get the original author's permission. A better option would be to take the central idea of the article and write a completely new article using your own text. If you're concerned that someone has duplicated your content, write to the website owner who has published your content requesting they remove the offending text. If the email does not elicit a response or your content then report the duplicate content issue to Google under the DMCA guidelines they provide at http://www.google.com/dmca.html
If all else fails, change your copy of the duplicated text. Keeping your copy fresh is essential so make the best of a bad situation and write even better copy.