Team Fortress 2 has been a test bed for Valve's ideas, one of which was to let users create their own in-game items. This has turned into a lucrative marketplace, in which some item creators have made a killing making items for virtual killing.

"Ten times as much content comes from the userbase as comes from us," Valve president Gabe Newell told students at the University of Texas, reported by PC Gamer. "We think that we're super productive and badass at making TF2 content, but even at this early stage we cannot compete with our customers in the production of content for this environment."

So how much has do the top-earners pull in? "The most anybody has earned in a single year is $500,000, so they're making content, selling it to other customers, and we have a revenue share with those people and their takeaway is $500,000." Newell also joked that Paypal had concerns about the cash flow moving through the system, and asked if the drug trade was involved. "We actually had to work something out with them and said, 'no, they're making hats.'"

Obviously, not everyone is becoming a millionaire off of TF2 items, but the fact that one person could make that much shows how robust the market can be. The booming in-game economy led to real-life transactions and, eventually, the hiring of an economist. Late last year, Valve launched a real money marketplace.