Knocking out or being knocked out by an opponent ought to be a viable tactic in an adventure game, but more often it is made too obscure or unwieldy in the rules of Dungeons & Dragons and its offspring. As an alternative (and as an encouragement in the use of this tactic), I offer the following house rule.
Any attack with a blunt instrument (either unarmed or with a weapon) can be made with the intent to knock out an opponent. The victim is entitled to a saving throw. If the saving throw succeeds, the victim takes no damage if a helmet is worn or half damage if the head is unprotected. If the saving throw fails, no damage is sustained, but the victim is knocked out and will remain unconscious for 1d6 turns or until revived. Opponents who are surprised suffer a -2 penalty to their saving throw; opponents who are attacked from behind suffer a -4 penalty.
Edged weapons with suitable non-edged surfaces (such as a sword's pommel or an axe's handle) may also be used as a blunt instrument for the purpose of this rule.