Which game is widely credited with popularizing the 'cosmetic microtransaction' business model for free-to-play games, allowing players to purchase purely aesthetic items without gameplay advantages?

The gaming industry pioneered innovative retail strategies that transformed how digital products are sold and consumed. From physical cartridges to downloadable content to microtransactions, gaming has continuously evolved its retail approaches. This poll tests your knowledge about a pivotal moment in gaming retail history that changed how the industry monetizes virtual goods and experiences.

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Team Fortress 2 (Valve, 2007)
World of Warcraft (Blizzard, 2004)
League of Legends (Riot Games, 2009)
Farmville (Zynga, 2009)