Two-Face's Coin

After Harvey Dent, the lawyer prosecuting mobster Sal Maroni, is splashed in the face with acid he becomes Two-Face. He then takes Maroni's lucky double-headed coin. Most famously, Two-Face uses the two-headed silver dollar, one side of which is scarred, the other side clean. Flipping the coin dictates Dent when making any significant life decisions. If the coin lands on the scarred side, Two-Face will pursue acts of evil. If the coin lands on the unmarked side, he is compelled to commit acts of good. Two-Face often defers to his coin in choices of life and death. At one point, Two-Face was so dependent on the coin for any of his criminal acts that he could be easily defeated by taking the coin away from him or preventing him from seeing the result of the flip. Harvey Kent (Earth-Two) was usually shown at this level of dependency. Dent has since lessened his dependency on the coin flip to whether a situation should be implemented, rather than each individual act.

Batman Forever
Two-Face's signature coin appears in Batman Forever. Batman easily defeated Two-Face by throwing up multiple quarters and silver dollars while the villain was in the middle of flipping his coin. Instead of being totally dependent on the coin for what he should do next, on at least one occasion- faced with an opportunity to shoot Bruce Wayne-, he decided what he wanted to do himself and just kept flipping the coin until he achieved his desired result (Although the Riddler stopped him from actually shooting Wayne, preferring to defeat him more directly rather than just catching him by surprise).

The Dark Knight
Two-Face flipped his own two-headed coin whenever he had to make a decision. Before his accident, Harvey Dent uses the coin to trick criminals into confessing, such as when he kidnapped Thomas Schiff and informed him, "Heads, you live. Tails, not so lucky". But because both sides were head, Dent would never kill. After his accident that turned him into Two-Face, one side of the coin was badly burnt while the other side was fine. He then used the coin to actually decide whether his target would live or die.

Behind the scenes

 * Some viewers have stated that in The Dark Knight, Two-Face wasn't dependent on the coin to make his decisions, and if the coin dictated that he couldn't kill his target, then he'd find a way around that. For example, when he met Sal Maroni in his car and the coin dictated that Maroni couldn't be shot, Two-Face shot his driver instead, causing Maroni to possibly die in the following car crash. Two-Face may have already thought ahead, however, and ensured that Maroni would live but would be severly injured and possibly unable to inform anyone of Harvey's actions.