Cooperative
Cooperative gameplay (often abbreviated as co-op gaming) is a feature in video games that allows players to work together as teammates against one or more AI opponents. It is distinct from other multiplayer modes, such as competitive multiplayer modes like player versus player or deathmatch. Playing simultaneously allows players to assist one another in many ways: passing weapons or items, healing, providing covering fire in a firefight, and performing cooperative maneuvers such as boosting a teammate up and over obstacles. Read the full article at the Wikipedia.
Zero-K
by Zero-K team
1.4.3.3
(12 Mar 2016)
Zero-K is a FREE, multiplatform, open-source RTS game.
Genre: Total conversion, Strategy 
HeXen II
HeXen II was the first game from an outside developer to ship with Quake engine technology.
Genre: Shooter, Action 
DOOM
by id Software
DOOM is a landmark 1993 first-person shooter video game by id Software.
Genre: Shooter, Action 
Alien Swarm
1.34 (31 Jan 2010)Alien Swarm is an overhead view tactical squad-based shooter for Unreal Tournament 2004.
Genre: Shooter, Total conversion, Action 
C-Dogs SDL
0.6.1 (24 May 2016)C-Dogs SDL is a 1 to 4 player overhead run-and-gun game.
Genre: Arcade, Shoot 'Em Up, Shooter, Action 
Zod Engine
by Nighsoft
2011-09-05
(6 Sep 2011)
Zod Engine is a remake of the 1996 classic game by Bitmap Brothers called Z.
Genre: Strategy 
Steel Storm: Burning Retribution
2.00.02966 (5 Dec 2011)Steel Storm: Burning Retribution (a.
Genre: Shoot 'Em Up, Action 
Chicken Invaders 4: Ultimate Omelette
Release (29 Nov 2010)I remember the day of the invasion well.
Genre: Arcade, Shoot 'Em Up 
Serious Sam II
by Croteam
2.070-RC2
(7 Nov 2006)
The series famous for relentless intensity is returning bigger, better looking and more frantic than ever before.
Genre: Shooter, Action 
Serious Sam: The Second Encounter
by Croteam
1.07-beta1
(14 Mar 2003)
The Second Encounter introduces three worlds where cyberpunk once again meets fantasy fiction, and features frantic action in 12 massive...
Genre: Shooter, Action 
Serious Sam: The First Encounter
by Croteam
1.05-beta3
(14 Mar 2003)
Serious Sam is a high-adrenaline action-arcade shooter heavily focused on frantic arcade-style single player action.
Genre: Shooter, Action 
Descent³
1.4.0b (25 Jan 2000)Descent³, the newest member of this classic 3D series, has an all-new game engine for improved rendering and gameplay.
Genre: 6DoF, Flying / Aviation, Shooter, Action