Just yesterday I was thinking, "I need a simple keyframe-animation tool." It's a thought I've had many times before, and every time I went looking, I'd find the usual things like Blender, AfterEffects, Flash, etc. They're all very advanced animation tools used mostly for specific disciplines. Each is also either too expensive for my teacher budget or too powerful for my budget computers.
Enter Wideo (is it Wideoo.com or Wideo.co?), a cloud-based, simple, keyframe animation tool. Despite its simplicity, though, it could easily be used to make some very complex animated videos, which would work as awesome supplements to ANY curriculum.
It's currently in beta, so it's still free. I'm nervous about what the price might be when they release version 1.0, but I'm hoping they offer some sort of teacher special. Speaking of, although I haven't made any yet, check out this video by a history teacher. This is what to strive for: