Here is an iPad app that will show you daily “deals” on apps you can use. Some are educational so this may be a resource to use in your classroom. You can see the Best Deals Today which has many apps discounted from $5.99 or less, down to FREE! There are also app mixes which allow you to download multiple apps for low prices and other features like “Spotted by users”. These apps are often free. It is updated daily so you can check to see what the newest links are.
You can also go to appgratis.com and see what is free for the day. In some cases, they have apps normally as high as $14.99 totally free for that day. You can also sign up for daily emails. He usually posts a free app and a discounted app.
You may not find useful apps everyday, but you may certainly find some apps that you can use either for productivity or use in the classroom.