Sutro Project / craigslist.org
In addition to maintaining the world's most popular list, the unflappable Craig Newmark operates the Craigslist Foundation "to help strengthen communities and spark community engagement."
There are no reported sightings of the headquarters open or even without the security cage - check out missed connections or casual encounters for the real Craigslist action :)
Address: 1381 9th Ave, SF, CA
Website: http://sfbay.craigslist.org/
Wikipedia: http://en.wikipedia.org/wiki/Craigslist
