Silk Road is an online marketplace that its operators run as a Tor hidden service. As such, visitors must use Tor to access the marketplace. While the majority of products that sellers list on Silk Road qualify as contraband, the site's operators prohibit goods or services intended to harm others. NPR has referred to the site as the "Amazon.com of illegal drugs."
This website plots some (hopefully interesting) statistics extracted from Silk Road. There's potential for much, much more data to be visualized but my time is limited and my interest finite. :)
If you think of something cool to do with all this, all data (scraped pages) and collation (JSON-generation) script are available over at GitHub