The first video game from Team Meat (Edmund McMillen and Tommy Refenes) of Super Meat Boy fame, Grey Matter is billed as an "anti-shooter", due to its inversion of Collision Damage - the enemies do the shooting, and it's your job to ram into them. The theme and general aesthetic of the game is mental illness, but that's really just there to compliment the craziness of the gameplay.
The original can be found here. A slightly updated version was also released as part of McMillen's Basement Collection.
This game provides examples of:
- Boss Rush: An odd example; Boss mode only has one boss, but the game tracks the time it takes you to beat it, and gives you 100 lives to do so with. And since it makes you do it with minimal upgrades, you'll need a fair number of them.
- Bullet Hell: The final boss in particular, especially on Expert mode.
- Collision Damage: Notably inflicted by the player.
- Evil Is Visceral: The enemies all consist of brains and pieces of bone.
- Flash Step: One type of enemy will avoid you this way. The most reliable way to kill them is to back them into a wall.
- Frickin' Laser Beams: Both the final boss and some late-game enemies use these.
- Rule of Three: Defeating three of the same enemy type in a row will increase your chain multiplier and unleash a trinity attack, which is a triangle with vertices at the three enemies' death positions.