Team Fortress 2

Team Fortress 2 arrived in 2007 as part of The Orange Box, built on the Source engine. Nine classes with sharply different roles — from the fast Scout to the lumbering Heavy — made it one of the longest-lived team shooters ever, with a cartoon style that has aged better than its realistic contemporaries.

In 2011 the game went free-to-play and Valve added the Mann Co. Store: cosmetic hats and items that could be bought, dropped and traded. That "hat economy" became an early, almost notorious model for microtransactions and virtual markets — items gained a real-money resale value long before that was standard.