I cannot believe it has taken me this long to cover Showbie. We just discovered this app during a PD meeting a few weeks ago. I immediately instituted it in my classroom, trained the students how to use it, and have not looked back since. It allows for incredibly easy document distribution to students and paperless assignment collecting/grading for teachers. It is intuitive, easy, useful, and pretty much free to use.
This app is absolutely essential for any 1:1 iPad classroom at any and all grade levels. I feel like a total iPad amateur for having gone a whole year without utilizing this app before now.