Craigslist was recently valuated at a cool $5 billion.  That's $5 billion for a site that looks like this.  Silicon Alley Insider covers puts their spin on the recent valuation here and does a good job at applying some science to the numbers. 

2007 Est. Revenue: $55 million
2008 Est Revenue: $81 million
Monthly Pageviews: 9 billion
Monthly Job Listings: 2 million   
Monthly Ad Listings: 30 million
Employees: 25

You may recall that YouTube sold for $1.7 billion to Google not long ago.  Is Craigslist worth three times that?  Debatable.  Their old-school classified engine format is still working, but for how long? If I was Craig Newmark, I don't know how long I could keep looking at those valuation numbers and not open things up to the highest bidder. There's got to be another online marketing challenge he can tackle with that much startup cash.