Most weapons that inflict lethal damage can be used to inflict nonlethal damage if the wielder takes a -4 penalty to his attack roll beforehand. Learn more about inflicting nonlethal damage with weapons here: Nonlethal Damage.
To explain further, when a creature takes nonlethal damage
Do not deduct the nonlethal damage number from [its] current hit points. [This] is not "real" damage. Instead, when... nonlethal damage equals... current hit points, [the creature is] staggered (see below), and when it exceeds current hit points, [it falls] unconscious.
If a creature's nonlethal damage is equal to his total maximum hit points (not his current hit points), all further nonlethal damage is treated as lethal damage.
So a dude has to really be beating on the fool to kill him with nonlethal damage. For example, a creature with a maximum of 10 hp who takes 11 points of nonlethal damage takes 1 point of lethal damage and is unconscious. Then the dude would have to continue beating the unconscious foe for 9 more points of damage plus the unconscious creature's Constitution score to kill him.
Unless the dude's whipping out his Monster Shark Turbo Strike or whatever and using it for enough nonlethal damage to both A) equal the foe's maximum hp and B) equal the foe's remaining actual hp plus the foe's Constitution score, the foe should be fine in a few hours.
You heal nonlethal damage at the rate of 1 hit point per hour per character level. When a spell or ability cures hit point damage, it also removes an equal amount of nonlethal damage.
Aside from that, there are weapons which specifically deal nonlethal damage by default, the list includes the sap, whip, bolas and others. With these, you take a -4 penalty while attempting to inflict lethal damage.
Finally, a character can improve her ability to inflict nonlethal damage using the following feats:
- Bludgeoner - no -4 penalty when using lethal bludgeoning weapons for nonlethal attacks.
- Stage Combatant - no -4 penalty when using (any) lethal weapon in which you have Weapon Focus. Despite the theatrical name, this is also useful for Paladins and any "merciful" PC with a signature weapon.
- Merciful Spell - a metamegic feat which allows a caster to cast a lethal spell as nonlethal (has no level increase).
- Betraying Blow - use Bluff for extra nonlethal damage while fighting 1 on 1.
- Sap Adept & Sap Master - allow a Rogue to deal a lot of nonlethal damage using sap and sneak-attacks.
- Pinning Knockout - allows a Monk to inflict a lot of nonlethal damage while grappling.
- Knockout Artist - extra nonlethal damage when combining sneak-attack and unarmed strike (useful for Rogue/Monk, Ninja and some other exotic combinations).
- Merciful Bane - allows Inquisitors (and others with Bane class feature) to switch their weapon to nonlethal damage while under the effect of Bane (no -4 on attacks, Bane extra damage becomes nonlethal too).