| Here are two ideas that I've used, and it has made my games a lot more fun.
1. Targeted strikes: This is like combining attack rolls with skill checks. The player character can, before his or her attack, declare an attempt to perform a particular task (e.g.: cut off an arm, knock off a helmet, disarm an opponent, knock an implement out of an opponent's hand, etc...). If the attack roll+modifiers exceeds the necessary total by a certain DC, then the desired effect takes place. This rule results mostly in an enhancement to the flavor of the encounter, but it entertains my PCs tremendously.
2. Critical Misses: A roll of a 1 on d20 always misses, but when using a ranged attack, if the trejectory of the attack passes through a square occupied by another creature (whether in front of or behind), a second d20 roll is cast. If the second roll ends up as a 20, the other creature is hit accidentally by the attack (be it friend or foe). This has made for some comical interactions between players, and makes flanking more risky. For melee attacks, I've kicked around the idea of having the attacker fall prone on a roll of 20 as a result of whiffing a little too hard. | 
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