Search around, there are several popular ones depending on your own ability and the amount of money you want to spend. I personally run the Cobb AP and love it, its a plug and play type. There is also the pensource route which has several good write ups in sticky form in the EM forum. Another popular one is the Utec, its a piggy back, a little more expensive, but can be tuned by yourself or a tuner also. Search around here some, find out which one you like and go from there.
"2007 WRX- PPG'd, 20G- Tim Bailey tuned
Stage 2 2005 WRX