Your Iphone must be jailbroken
You will need the Installous app from Cydia.
All you need to do is open Cydia then go to Manage/Sources/Edit/Add, then when prompted, type http://cydia.hackulo.us.
Return to Cydia and select Hackulo.us from the top menu under "entered by user", then scroll down and select Installous and press "install".
Voila! You can now install virtually any app for free from the apple store