I found this article particular interesting. In 2006, "Master Splyntr" (FBI agent Mularski) joined DarkMarket.ws under the guise of a spammer. He was eventually invited to work as a system administrator and for nearly two years the FBI gathered information about identity thieves and used the site to track the IP addresses and use of stolen information. DarkMarket.ws is one of many virtual marketplaces for identity thieves to broker skimmers, identities and even phishing schemes. The site was rather developed and included peer reviews of users and product reviews of items for sale.
Just recently the site was shutdown by the FBI and hopefully several arrests will be following.
I also found this snippet entertaining
The German report confirm rumors that have swirled around DarkMarket since late 2006, when uber-hacker Max Ray Butler cracked the site's server and announced to the underground that he'd caught Master Splynter logging in from the NCFTA's office on the banks of the Monongahela River. Butler ran a site of his own, and the warning was generally dismissed as inter-forum rivalry, even when Butler was arrested in San Francisco last year on credit card fraud charges, and shipped to Pittsburgh for prosecution.