Silk Road (marketplace)
The Silk Road in an online exchange that is only accessible as a tor hidden service. All transactions are done using bitcoins. It was recently written about in a Gawker article (http://gawker.com/5805928/the-underground-website-where-you-can-buy-any-drug-imaginable) which ended up getting wide coverage, even attracting attention from US Senator's and the Wall Street Journal.