Action
The action game is a video game genre that emphasizes physical challenges, including hand–eye coordination and reaction-time. The genre includes diverse subgenres such as fighting games, shooter games, and platform games, which are widely considered the most important action games, though some real-time strategy games are also considered to be action games. Read the full article at the Wikipedia.
Unearned Bounty
Unearned Bounty is an action-packed Free-For-All multiplayer game where you battle to become the most infamous pirate!
Genre: Arcade, Naval, Shooter, Action 
Dystopia
by Puny Human
Dystopia is a cyberpunk game on the Source engine that places the player into tense combat situations in a high-tech world spanned...
Genre: Shooter, Action 
Taric
1.1Taric is a game inspired by pac-man
eat all the food and beware of droids
Genre: Arcade, Action 
Gentrieve 2
1.12 (8 Jan 2013)Gentrieve 2 is a "Metroid"-style 3D action game featuring completely random & customizable worlds.
Genre: Shooter, Adventure, Action 
The Signal From Tölva
The Signal From Tölva is an open-world first-person shooter set on a distant, haunted, future world.
Genre: Shooter, Stealth, Action 
Infinity Runner
The Infinity is the largest spaceship ever built by mankind; flung into the void of space to secure humanity a new home amongst the stars.
Genre: Action 
Time of Dragons
by 4 I Lab
This is an MMO shooter where you ride dragons armed with missiles and lasers!
Genre: Shooter, Adventure, Simulation, Action 
Interstellar Marines
UPDATE 26 AVAILABLE - Interstellar Marines is an immersive tactical sci-fi First Person Simulator in the making, offering a unique...
Genre: Shooter, Adventure, Simulation, Action 
Sky Force Reloaded
Sky Force Reloaded is a classic shoot ‘em up experience packed in gorgeous visuals and excellent gameplay.
Genre: Arcade, Shoot 'Em Up, Action 
Chinbu's Adventure
A collectathon action rpg where you play as a young dragon named Chinbu with the unique ability to control all elements.
Genre: Adventure, Action 
Thinking with Time Machine
by Stridemann
Thinking with Time Machine is a Portal 2 mod in which you will find and use a Time Machine to create a twin of yourself from the past...
Genre: Puzzle, Strategy, Adventure, Action 
Synergy
by Synergy Team
Synergy is a simple Half-Life 2 Modification (Mod) that enables you to play through the Half-Life 2 series and many other 3rd-party...
Genre: Shooter, Adventure, Action