To find free apps, a good place to start is to just go in the iTunes store and view all of the top apps in each category. There's a free column and a paid column. There is also an app I like called AppShopper, it lists the apps that have price drops. A few apps are free for a day or to sometimes, and I've gotten a few decent ones for free that way, too!
I have over 100 apps, and not one of them did I pay for!
Sent from my iPad 2 using iPF