I've written about Voice Thread quite a bit in the past as a tool for connecting classrooms and for involving parents and grandparents in student projects. Voice Thread is a great tool for hosting online conversations about pictures and or documents. Voice Thread can be used to connect classrooms around the world to create a global conversation.
Recently, Colette Cassinelli added a new element to her VoiceThread for Education wiki designed to help teachers find other teachers who would like to connect their classrooms. If you haven't checked out Colette's wiki, I encourage you to do so. The VoiceThread for Education wiki has many excellent ideas for and examples of using VoiceThread in education. There is something for everyone in the VoiceThread for Education wiki, even mathematics teachers.