Something about Voicethread captivated me when I first saw it, probably two or three years ago. But only recently have I tried to use it in the classroom. Freshmen were learning about the Trinity. I pulled a variety of images of the Trinity off the web (with links to the sources) and put them in a Voicethread slide show. Students chose an image and then recorded a comment in which they explained how the image connected to concepts they had learned in the unit.
It was a way for them to show understanding. I suppose it could have been done in a written form. But it was cool this way, and students seemed to enjoy the assignment. What I need to find out now is if anyone listened to what their classmates had recorded, since that was possible but not required.
I've got in my head another assignment, involving characters from Luke's gospel and a student choosing one and recording as though she is the character. But since there is some setup time involved and we're nearing the end of our time with Luke, that might have to wait until next year.