Call of Duty/Mods

In its heyday, Call of Duty had a bustling multiplayer packed with servers and mods. Time has taken its toll though; there are typically around 200-300 populated servers a day, most with less than ten people.

Mods

Additional War Effects (AWE)

By far the most popular and enduring mod today, Additional War Effects ramps up the intensity with a bunch of added functionalities, including:

• Cookable grenades. To cook a grenade, simply hold the fire button with a grenade armed. A bar will appear showing the progress of the cook, which is around five seconds. Don't hold it too long, or else you'll blow up. For balance reasons most servers limit soldiers to only one grenade.
• Sprinting. To sprint, press and hold F. You can sprint for around five seconds.
• Spawn protection. It is denoted by a cross over one's head. If you move, your protection will disappear; if you stand still, it will persist for around five seconds. It is advisable to stand still a bit and check your surroundings before moving.
• Dropzone. On many AWE servers, a team (in team deathmatch) or a select few individuals (deathmatch) will spawn parachuting into the battlefield. Spawn protection is offered around half-way down, but expect to be killed very quickly.
• Bleeding. If someone is shot too many times without dying or is shot by a bolt-action rifle without dying, a player will bleed. Depending on the severity of the attack, their health will slowly drain away unless they reach a healthpack.
• Player shouts. Although they differ from server to server, they usually involve taunts after a kill ("Adios amigo!" for Americans, "Sig heil!" for Germans, "Have it!" for British, and "Down with fascism" for Russians) or cries of pain upon bleeding.
• Increased chance of shellshock. If you are too close to a grenade blast, fall off too high of a height, or are damaged by a melee attack, you will enter shellshock.
• Hit detectors. On some servers, if you hit someone, a circle will flash around your crosshairs. Very useful to see how wounded someone is.
• Screen changes. Upon a "closeup kill", blood will temporarily splatter on your screen. If you are headshotted, there will be a bullet hole in your screen.
• Looting bodies. If you press and hold shift over a dead body, you will be able to uncover their other equipped items: alternate weapon, pistol, and grenades.

Because it is server-side, you do not need to install it in order to play a game. You do need to install if you want a server with AWE though.