Which game was the first to successfully implement a 'hat economy' that generated millions in revenue from purely cosmetic virtual items, revolutionizing in-game purchase models?

The gaming industry has revolutionized digital marketing and customer loyalty through virtual economies and in-game purchases. These innovations have created entirely new revenue streams and engagement models that traditional retail could only dream of. Test your knowledge about one of the most successful early experiments in virtual goods that helped shape today's gaming marketplace and loyalty systems.

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Team Fortress 2
World of Warcraft
Second Life
Counter-Strike: Global Offensive