Team Fortress 2 - Did You Know Gaming? Feat. Markiplier
Quake was first conceived as an adventure or RPG game, similar to The Legend of Zelda. Quake was to be the name of the player character, and he would wield a massive hammer, which would increase in power as he defeated enemies. By the time the development team had completed the engine and networking code, they were burnt out on the project. Uncertain that they could meet their release date, they held a vote and decided to change the gameplay to something they were familiar with: a first-person shooter, like their earlier release Doom.
Contributed by Spherix
In the Quake 1 Manual, it states that Grunts have probes inserted into their pleasure centres, and whenever they kill someone, they feel pleasure.

"Goons with probes inserted into their pleasure centers; wired up so when they kill someone, they get paroxysms of ecstasy."
Contributed by retrolinkx
The graphics for the nailgun ammo contains the logo for Nine Inch Nails.

This is a reference to Trent Reznor, who did the original soundtrack.
Contributed by Patient Zero