Happens all of the time and usually there is not much you can do about it. If they are not getting traffic or stealing your rankings you shouldn’t worry to much about it.
Now with that being said you do have some options:
1. Call or email them directly and ask them to stop.
2. Call their hoster and explain the situation. They usually don’t take kindly to this stuff and will ask them to take it down
3. Contact the search engines through DMCA fillings:
Digital Millennium Copyright Act
4. More WPW threads on the subject:
Someone finally copied my website!
http://www.webproworld.com/google-discussion-forum/57919-has-anyone-had-real-success-googles-dmca.html
Yahoo, The DMCA, And Blackhat Techniques
Website Content - Stolen?
http://www.webproworld.com/google-discussion-forum/19260-stolen-website-content-detection.html