A list of top marketplace-centric companies was released today. It's called the A16Z Marketplace 100, and it ranks the top 100 companies that are marketplace startups and/or non-public companies. On top of this list are four titans - all four of them relatively new companies - followed by a fifth... Valve. The top four companies on this list account for a whopping 76 percent of consumer spend. That's a dominance we don't often see in any industry... unless we're talking about the smartphone market, of course.