Which gaming company first introduced the concept of 'microtransactions' in a major mainstream title, revolutionizing the gaming retail model?

The way gamers purchase and access content has fundamentally changed over the past decade. In-game purchases, downloadable content, and virtual economies have transformed gaming from a one-time purchase model to ongoing revenue streams. This trivia question explores a pivotal moment in the digital transformation of gaming retail that changed how developers monetize their products and how players consume content.

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Valve Corporation (with Team Fortress 2's hat economy in 2010)
Electronic Arts (with FIFA Ultimate Team in 2009)
Blizzard Entertainment (with World of Warcraft's in-game pet store in 2008)
Bethesda (with The Elder Scrolls IV: Oblivion's Horse Armor DLC in 2006)