Blind-Fight

In melee, every time you miss because of concealment, you can reroll your miss chance one time to see if you actually hit. An invisible attacker gets no advantage to hit you in melee. Thus, you don't lose your Agility bonus to AC, and the attacker doesn't get the usual +2 bonus for being invisible. The invisible attacker's bonuses do still apply for ranged attacks, however. You take only half the usual penalty to speed for being unable to see. Darkness and poor visibility in general reduce your speed to three-quarters normal, instead of one-half.