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.
Chaos Projectile
Chaos Projectile is an open source 2-dimensional action arcade game with role-playing elements, currently in development.
Genre: Arcade, Beat 'Em Up, Action 
Strife: Veteran Edition
Hailed as the original FPS-RPG game that spawned countless imitators, Strife: Veteran Edition is a love letter with new features to...
Genre: Shooter, Action 
Veraball
A "roll the ball" game. Collect coins, then reach the goal as fast as possible, without falling out of the level.
Genre: Boardgame, Action 
Tuxemon
Tuxemon: An open source attempt at creating a Pokémon work alike engine in python using the pygame library
Tuxemon is a completely...
Genre: Action 
Reaction
Reaction is a free-to-play first person shooter that combines action and realism; with a specific emphasis on action over realism in...
Genre: Shooter, Action 
OpenGOO
OpenGOO is a free and opensource clone of World of Goo game, written in Qt 5 and Box2D as a 2D physics engine.
Genre: Puzzle, Action 
Petigor's Tale
This is a game about a brave warrior - a teenage boy who alone was forced to fight an evil troll and his companions.
Genre: Beat 'Em Up, Action 
ReTux
ReTux is an action platformer loosely inspired by the Mario games, utilizing the art assets from the SuperTux project.
Genre: Platform, Action 
Jump'n Bump
In this game you must jump on the opponent with your rabbit to win.
Genre: Platform, Action 