Back in the good old days of 2013, Amy Phillips blogged about both Apps Gone Free and Appshopper.com.
App developers and publishers are constantly changing the price of apps in the iTunes App Store, and Apps Gone Free is a great way to find apps that have, well, gone free for that day. It combs through the App Store finding apps in various categories whose prices have dropped to 0 for the day or even week. This is a great way to build your library. Though, if an app does go free, and you want it for your whole class, you'll need to have them download it that day.
Sometimes these apps will stay free for a while, but sometimes it's only for one day! If you missed an app that went free, Appshopper.com is a great way to track the price of an app you like. You'll need to sign up for an account. Once you do, you can search for the app on the site and add it to your wish list by clicking 'want' on the right hand side.
In your settings, you can also set up how you would like to be notified. If a price changes on one of my tracked apps, I have it set to send me an email.
Both of these apps are fantastic ways to track those handy free apps you may have your eye on!