Somewhere in my archives from awhile back is another link I posted to a site listing a great many instances like these listed here: 

Call for Civilian Backup – Patrol Tactics Channel – POLICE Magazine.

Fortunately, there are those civilians who—requested or not—will jump into the fray in a cop’s hour of need. People like twenty-one-year-old Ben Saks, who in 2006 was shot in his left hand while helping a police officer. Or Texan Travis Neel, who, having witnessed the shooting of a Harris County deputy sheriff, pulled his own gun and fired, driving the deputy's assailants away. Then there’s Ralph Festavan, who watched as a heroin peddler attacked a Shreveport (La.) policeman and grabbed the officer's gun. Festavan ran to a nearby patrol car and grabbed a shotgun, which he used to shoot and kill the offender. I should also mention Floridian Vincent McCarthy, who didn’t hesitate to lend help to a police officer struggling with a man and woman at the side of the road. When his own efforts failed to deter the man’s assault on the officer, McCarthy shot the cop’s attacker once in the leg with a pistol he was licensed to carry, stopping the attack. These and many other citizens have put their own butts on the line to go out of their way to assist officers, often saving their lives.

Also recall this case from Baton Rouge:

Stephens ordered Harrison's attacker to stop the attack and get off the officer. The verbal commands were ignored. The armed citizen fired four shots, all of which struck Temple in the torso. With Temple still on top of the officer, Stephens advanced toward the struggle. He again ordered Temple to stop the attack, and get off the officer. Those commands are ignored. Stephens then fired a fifth and final shot into the head of George Temple