AppsGoneFree app and love it! Each day, it lists apps that are free for that day or for a limited time. I check it every day and there are great apps going free regularly. Some are educational, some are photo apps, some are games, but you can check them out and only get the ones you think you will use (or your students will use!)
I have started posting on Facebook and Twitter the apps that are free and include the link from iTunes to share with my friends and PLN when I know about free apps. I've had lots of comments telling me they look forward to my free app alerts, so if you do this, I encourage you to SHARE AWAY!