After originally purchasing apps through iTunes vouchers and then gifting apps with a credit card, now that it is available, we are finding Apple's Volume Purchasing Program very useful. While purchasing a licence for every app on every iPad is expensive, it is the only way to be Apple compliant as an educational institution. With the VPP, there is a 50% discount when you buy 20 or over. Purchasing of apps is done centrally but then the codes are used to redeem the app on each iPad separately. Teachers are then able to download free apps as they wish. Purchasing apps this way, because we obviously need to stick to a budget, also forces teachers to be more creative in the way they use the available apps.
Additionally, Apps Gone Free is a free app that I use to look out for apps that are free for a day or for a short period of time.