iOS app prices change all the time. Many times the publisher will make an app free for a short period of time... maybe a day, maybe a week. "Buying" these apps when they're free is a great way to build your app library cheaply!
There are lots of websites and apps to help you find temporarily free apps. One of them is the app "Apps Gone Free". Apps Gone Free searches the App Store daily so you don't have to! They pick out a few apps that meet certain requirements: 3+star or better rating, don't use outside ads and haven't been free over and over again. Each day the featured apps are a mix of games, productivity and utilities.
Do you have a favorite way to find free apps? Add it to the comments!