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
The third way to get paid apps for free is by actually going into the Starbucks store and getting one of the little cards that give away free songs or apps. Sometimes these are the best apps you can get for free. I suggest downloading them even if you don't know that you will like it because even if you delete the app it will still be saved in the cloud. If you change your mind you can always download it again after deleting it and it will still be free. :)
Enjoy your free apps! Keep Life Simple, get stuff for free. :)