Combat Reflexes
In my experience, this feat is only useful against stupid creatures or things that happen to be fleeing. A clever combatant can avoid attacks of opportunity with ease, and unless your foes are using some strange tactical maneuver (such as sending a heavily armored guy to draw an AoO and then allowing a lightly armored guy to move past), it's not that good. Furthermore, you only receive a number of additional attacks equal to your Dexterity bonus.