Expanding on the example of Trip, Creature B doesn't have Improved Trip, but Creature A does, and he has Greater Trip, which lets you make Attacks of Opportunity on your opponent once you Trip him, so A Trips B, and A takes his attack of opportunity. There is another feat, Vicious Stomp, which lets you take an attack of opportunity on someone if they go prone. This stacks with Greater Trip, so if A Trips B and has Vicious Stomp AND Greater Trip, he gets 2 Attacks of Opportunity.
I love Attack of Opportunity Builds.
In a sense, though, you can't perform Attacks of Opportunity during your own turn. Attacks of Opportunity technically happen outside your turn even when they happen during your turn. So, if your Tripping Character, the Red Scare, were wielding 2 weapons, a Hammer and a Sickle, and he has 2 weapon fighting and Greater Trip. As he performs his Full Attack Action, a Full Round Action, he attacks with both weapons at a -2 for 2 weapon fighting and off-hand weapon is light. Let's say his off-hand attack with the Sickle is a successful Trip, and The Red Scare now gets an Attack of Opportunity with the hammer. That AoO does not suffer the -2, because AoO's happen outside the round.
