Check out this site called VOICE THREAD
It's a collaboration site for all types of settings, including the classroom. You can collaborate with your voice and text. Click on the classroom link to see demos of Bell-work/Warm-ups, Venn Diagrams and a photo exercise.
Here's another one that is similar to voice thread called QlipBoard http://www.qlipmedia.com
Here's a site where kids don't need computers at home to participate: