All starting apps on your iPhone and all apps that you downloaded for free are free to use. Though, not all apps are not free from the beginning, all apps that you have downloaded are free for you to use as long as you want.
If you have an Iphone you can find free mobile apps from the Apple Apps Store. There is an app on the Iphone that will take you directly to the Apple Apps store so you can get free apps straight from your phone.
There are tens of thousands of free apps from the App Store. Also, if you have a jailbroken iPhone, you can get hundreds/thousands more free apps from Cydia. Also, check this website frequently to get all the recent free apps from the app store: SEE LINK BELOW: