There are 3 ways that I know how I normally build my paid app database for free. Sometimes, even if I don't want the app right now but maybe will want it later when I get an iPad, I will still download the app.
First way is to go to the App Store. Every week they give away an app that you would normally have to pay for. This way you get a free app to add to your device. Sometimes these are really great, sometimes not. Depends on your preferences.
Second way to recieve paid apps for free is to