Impact

An Impact weapon is more likely to make critical hits.

Additional information

 * The Improved critical feat can do the same for all weapons in the matching category, however, those two effects do not stack.  An Impact Warhammer, for example, has a threat range of 19-20, but does not have a threat range of 17-20 if it's owner have taken the Improved critical feat.
 * The Keen weapon enhancement doubles the critical range for piercing and slashing weapons.