Doom
A space marine stationed on the moons of Mars battles hordes of demons and undead soldiers who have overrun a research facility following a disastrous portal experiment. The game's pseudo-3D engine created a visceral sense of space and movement that felt categorically different from anything that had come before it. Its non-linear level design and hidden secrets rewarded thorough exploration.
Doom popularized the first-person shooter genre, 3D graphics rendering, and networked multiplayer deathmatches, fundamentally transforming the PC into a serious gaming platform. It is estimated to have been installed on more computers than Windows 95 in 1995, and its open modding system — which allowed users to create and share their own levels — established the modding culture that would produce Counter-Strike, Team Fortress, and DotA.
id Software released Doom as shareware in 1993, allowing anyone to download the first episode for free — an early model of digital distribution that Microsoft actually cited as a threat to its new operating system. The game's violent content made it the centerpiece of a congressional debate about video game violence, directly leading to the creation of the ESRB ratings system.
Ubiquitous broadband internet shifted the medium from solo play to massive persistent social worlds, creating entirely new genres and business models. The MMORPG, the competitive shooter ladder, and eventually the live-service model all emerged from this transformation of games into always-online social spaces.