Smart Apps for Kids is ahhhhhhmazing. I kid you not! Every day they feature a Free App of the Day (generally for preschool or elementary but sometimes older). But most days they feature ridiculous amounts of free and discounted apps. The apps are not theirs - they scout out what is free right now and tell you about it! Most days there are several apps listed and on Fridays they host Free App Friday, which often features over $100 in free apps! The apps they feature are sometimes free for that day only or a few days only, so be sure to check them out and grab what you like while it is free!
In my classroom, I have 4 iPads (and typically about 22 kids). We use them for projects, etc., but when we have indoor recess, the kids have free time on the iPads on a rotating schedule and I need to have good stuff on there that they can use independently but is also educationally relevant. I don't have an app budget at all... zero dollars... so Smart Apps for Kids has been a lifesaver! I subscribe to them via email and Twitter so that I don't have to remember to go and check every day to see what's new.
Also... and here comes the second thing You Oughta Know... did you know that if you have multiple iPads, you can download something on one of them and have it automatically download on your others? It's TRUE! Go into Settings and on the left hand side scroll down to iTunes & App Store. You can see it hilighted on the left in the picture: