Curriki, the online education community, is building the first website to offer free, open-source instructional materials for K-12. We have thousands of free worksheets, lesson plans, exams, project ideas and activities for English language arts, math, science, social studies, technology integration and other subjects. All of our educational material is contributed by teachers and partners and is free and open source.
I LOVE VoiceThread. I think it is super easy to use and provides students the opportunity to express themselves digitally.
It works great for elementary students all the way up to high school students (even adults).
You can create a single educator account for FREE or get a classroom subscription. Depending on how often you will use it I recommend splurging and buying the class subscription because you can create individual student accounts.