http://www.craigslist.org 

From the website (links mine)... 

 CraigNewmark? observed people on the Net, on TheWell and in UseNet, helping one another out. In early '95, he decided to help out, in a very small way, telling people about cool events around San Francisco like the Anon Salon and Joe's Digital Diner. It spread through word of mouth, and became large enough to demand the use of a list server, majordomo, which required a name. [...] 

 Over time, people started posting items on the list in different areas, jobs, stuff for sale, and apartments, the latter in response to San Francisco's apartment shortage. Craig wrote software which could automatically add email postings to a site which became www.craigslist.org. [...] 

What works 

Giving people a voice 
A sense of trust and even intimacy 
Consistency of down-to-earth values 
Simplicity 
No charges, except for job postings 
Freshness of the material 
No ads, particularly no banner ads 

 craigslist won the 2001 WebbyAward? for best OnlineCommunity. 

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CategoryOnlineCommunity