Though not free ($1.99), this is almost like an entry level screencasting app where students draw and then tell the story about their drawing. Completed projects can be shared.
Animoby is a sort of screencasting tool, but not really. The idea is to instantly create and publish products that include voice, drawings, and images.
Though $4.99, this app allows you to annotate video. For PE teachers, coaches, music teachers, science teachers or anyone else looking to make notes on physical actions
Great example of combining the physical and the digital. Students took pictures of paper projects and then used Explain Everything to explain their dioramas.