Prerequisite: Combat Reflexes, Improved Unarmed Strike.
Benefit: Whenever an opponent falls prone adjacent to you, that opponent provokes an attack of opportunity from you. This attack must be an unarmed strike.
FAQ
The Greater Trip feat allows you to take an attack of opportunity against a foe that you trip. The Vicious Stomp feat allows you to take an attack of opportunity against a foe that falls prone adjacent to you. If you have both these feats and trip a foe, do you get to make two attacks of opportunity (assuming that you can)?
Yes, the two triggering acts are similar here but they are different. One occurs when you trip a foe. The other occurs when a foe falls prone. It requires a large number of feats to accomplish, but you can really pile on the attacks with this combination.