Scribblar is an online whiteboard application. While many disciplines lend themselves well to text, there are certainly classes where the ability to literally draw students a picture is worth a thousand words.
This is a free online whiteboard where students can collaborate on chemistry and math problems without worrying about typing symbols. Also students can work on group projects together.
Probably a site most have heard of before, but well worth the look if you haven't. Basically, it's one man who sits at his computer with a slate and narrates videos explaining an idea/technique as demonstrated on a simple whiteboard. He's recorded a large compilation of these videos from a variety of fields in math and science.
"The Khan Academy is a not-for-profit 501(c)(3) with the mission of providing a world-class education to anyone, anywhere. Despite being the work of one man, Salman Khan, this 2100+ video library is the most-used educational video resource as measured by YouTube video views per day and unique users per month."
Explain Everything is an easy-to-use design tool that lets you annotate, animate, and narrate explanations and presentations. You can create dynamic interactive lessons, activities, assessments, and tutorials using Explain Everything's flexible and integrated design. Use Explain Everything as an interactive whiteboard using the iPad video display (via Airplay/cable).
I found this plug-in for Moodle that allows real-time sharing of multi-media such as chats, your screen/desktop, whiteboards, presentations, and audio/video. Appears to be an option for incorporating synchronous activities and communication.