Copyscape is really an excellent online technology tool which tells you the websites which have duplicated the content of your website. Just type the domain name of your website and you will get the list of websites which have copied the content of your website. And that for free. It has a premium paid service which has lot more features such as automatic search of plagiarist, check whether the content you are uploading is original or not, placing copyscape banners to prevent duplicating by others etc.. Just check this video to get the whole idea.
Another online service which is similar to copyscape is Blekko.