implemented
SRD
Prerequisites: Str 13, Power Attack.
Benefit: If you deal a creature enough damage to make it drop (typically by dropping it to below 0 hit points or killing it), you get an immediate, extra melee attack against another creature within reach. You cannot take a 5-foot step before making this extra attack. The extra attack is with the same weapon and at the same bonus as the attack that dropped the previous creature. You can use this ability once per round.
Special: A fighter may select Cleave as one of his fighter bonus feats.
In our game
Cleave is automatic. When a normal melee hit drops a creature below zero HP, or destroys a creature whose death threshold is zero, you make one immediate attack against the nearest eligible hostile within clear reach. It uses the triggering weapon and attack bonus, with a new roll against the second target's defenses. You do not move or change your selected target. The follow-up cannot trigger another Cleave, and damaging an already dying creature does not trigger it again.
The current one-normal-attack-per-round system permits at most one Cleave every six seconds. Choosing Power Attack earlier and having Strength 13 unlock the feat. The runtime contract and verification review record the supported behavior; the adaptation is cleave-target.
Open Game Content from the System Reference Document 3.5, used under the Open Game License 1.0a.