To get your content removed from the another website, the first step (that works in 99% of the cases) is to send a friendly email to the owner of that site.
But sometimes finding the email address of the person, who ripped off your content, may be the toughest part. Here are some indirect tricks to help you locate email addresses of offenders.
Step 1. If the offending site is self hosted, you are very likely to find the email address of the website owner through the public whois database. I prefer DomJax for quick retrieval of website registry information.
If the website is self hosted but the whois information is private, try the following query on Google (replace xyz with the actual site address) xyz.com -inurl:xyz.com - This might show up some real email addresses for that domain.
Step 2. If the site is hosted on Blogger, WordPress or other blogging platforms, leave a comment in any of the blog posts. Most bloggers generally read and respond to user comments.
Step 3. Now the trickiest part - there's no "Contact Us" form on the website and there's place for you to write comments. Well, don't give up yet because you still have good options:
3(a). Use Technorati or Google Blog search to find other sites / blogs linking to that offending site. It's quite possible that the offender may be maintaining multiple websites that are cross-linked for maximum Google Juice. There are chances that contact information is available on one of his/her legit sites.
3(b). If the person is blogging under an alias (say "PeterCool008") - search for that term on Google. It's very likely that he has used the same alias when registering on other web forums and social networking websites.
See: Writing DMCA Complaint to Google
Never quit - I know it takes some effort to track down the plagiarists but that should not deter you from actively pursuing your case. Remember that search engines, including Google, still have issues figuring out the original source of web documents.