Many of its lawsuits arise, not from articles posted by a website's proprietors, but from comments and forum posts by the site's readers. Under the Digital Millennium Copyright Act, a website enjoys effective immunity from civil copyright liability for user content, provided they, promptly remove infringing material at the request of a rightsholder. That's how sites like YouTube are able to exist, and why Wired.com allows users to post comments to our stories without fear that a single user's cut-and-paste will cost us $150,000 in court.
...
“The DMCA is a good deterrent from being sued,” says Kurt Opsahl, a staff attorney with the Electronic Frontier Foundation, “Complying with conditions of eligibility for the safe harbor is a good thing to do. It probably will prevent somebody from suing you in the first place.”
...
If you run a U.S. blog or a community site that accepts user content, you can register a DMCA agent by downloading this form (.pdf) and sending $105 and the form to Copyright RRP, Box 71537, Washington, D.C., 20024.

It's like a force field for lawsuits.

Read More http://www.wired.com/threatlevel/2010/10/dmca-righthaven-loophole/