Copyscape is so far the best tool (it is a website) I have ever used to help detecting duplicate content of your website. I've been using this tool since I started blogging in few years and recently they made a video of it so now you can watch the following video for the introduction of Copyscape:
This is usually how I will you use the Copyscape once in a while to detect any losers trying to steal my content. Of course I used it for free without paying anything. This is what I did recently:
(1) I visited Copyscape and entered "http://softwaretracker.blogspot.com"
(4) I sent an email to this fella with the tittle - "Urgent: Copyright infringement on your website (www.windvis.com)". I politely asked him to remove the content immediately and if possible without forcing me to file DMCA complaint with his web host.
P/S: What really surprise me is that Google gives this website Page Rank 3? Wake up, Google! :) I think the search engine should be smart enough to detect plagiarism. What do you think?