CraigsList is the 7th most trafficked site on the internet, with only 19 employees, because the users do most of the work themselves. Built on top of open source, and using open source principles to build its data and services, CraigsList is reshaping the marketplace of classified advertising. But because advertising on CraigsList is mostly free, it's not taking the revenue from competitors, just changing all the rules by which those competitors do business. This is a classic example of asymmetric competition. Sites like CraigsList that are built via user self-service and an architecture of participation provide a huge challenge to existing business models, and a huge opportunity to reshape the economic landscape of their industries.