Collaborize classroom looks very interesting and it's free. Encourages student engagement, shares lesson plans and more. Pretty strong endorsements of website as well. Worth checking out.
VoiceThread is a collaborative tool that allows collaboration online. Responses may take the form of video, audio, doodle (drawing) or typing) and also phone (scheduled call which will record). Many ways to accommodate learners. Highly recommend. Free, but I upgraded to Pro version.
Great site for making mind maps and doing online brainstorming. Able to be exported and shared with others. Great tool for teacher collaborators and for kids! Super easy to use
Notability is an iPad app for note taking that allows collaborative (or not) notetaking with sharing options. Plus it works with Dropbox (cloud storage)