Below is a promotional video from Atlantis, which describes itself as "the world's best anonymous online drug marketplace."
Unsurprisingly, Atlantis accepts Bitcoins and Litecoins. For those familiar with Bitcoins Atlantis may sound similar to the website Silk Road. The CEO of Atlantis did an AMA on Reddit where he/she explained (among other things) why he/she thinks Atlantis is better than Silk Road.
Like Silk Road, to use Atlantis you will need to download the Tor browser bundle, which will allow you to browse the internet anonymously. Nick Gillespie recently sat down with Tor's development director to discuss how Tor works, how it started, and more.