If you don't subscribe to the Smart Apps for Kids FREE App Alerts, I would recommend that you do so ASAP. They send a weekly email with lists of apps that are free, and descriptions about each app. Really cool and helpful.
This week they compiled a list of the Top 50 Totally FREE Kids Apps. This list is FANTASTIC and features many of my classroom favorites. CLICK HERE to read the list and download the apps from the App Store.