I usually deal with 5-10 cases of copyright infringement per week. Most of these are posted on free sites like MySpace. At that rate it's important to be fast. Just send a DMCA notice to the host, and let them handle it. This will normally get the content taken down within a few days, and it won't waste your time.
To find a site's host, you can use:
Complete Whois
Some links to report abuse for common domains:
Blogger-Blogspot Report Abuse Microsoft & Live Spaces Report Abuse Myspace Report Abuse With practice you can complete and fire off a DMCA notice within about a minute.