For those of us who tutor via Skype, it's vitally important that we have access to an online whiteboard. A physical whiteboard isn't nearly as convenient: it would have to sit behind me in the webcam view, and I'd have to shift out of my chair every time I wanted to use it. The student might also want to see the notes on the board, and would have to ask me to move. While I definitely need the exercise, I don't think all that standing and sitting and standing is productive in a class context.
Just today, on my Twitter feed, I saw someone link to a new, super-stripped-down type of online whiteboard, called very simply "A Web Whiteboard." No installation necessary-- not that this is unique to A Web Whiteboard, but it does make the app more convenient. I tried the app out for a few minutes... and came away a bit disappointed. While AWW has the advantage of being cross-platform (you can use it just as easily on your smart phone or iPad as you can on your computer), it's missing features that are vital to tutoring, such as the ability to write text or to create clean geometric shapes when visualizing math problems. As far as I can tell, and I may be wrong about this, the whiteboard is geared entirely toward freehand work.
A much better whiteboard for tutors is, in my opinion, Dabbleboard. Dabbleboard is almost as easy to use as AWW is, but it comes with far more features. You can write text on Dabbleboard, for example, and can adjust the size of the text. The text palette includes simple mathematical symbols (pi, square root, some exponents, etc.). You can draw freehand or use a shape-recognizing "orthogonal" tool to create geometric shapes.* The board's animation quality is also much better than AWW's. Dabbleboard includes a video chat function (is this a recent addition?), which makes Skyping a bit redundant, but since I'd normally have to "meet" my students via Skype before referring to the whiteboard during a session, it's better for me simply to remain with Skype.
All in all, if you're an online tutor, I can't recommend AWW in its current state. Once AWW adds more features, I may change my tune. But for now, of the various online whiteboards I've tried, Dabbleboard is the best fit for me.
*Unfortunately, Dabbleboard interprets your attempts to draw trapezoids as attempts to draw rectangles. To make a trapezoid, you have to draw each line segment separately-- a difference of only a few seconds, so it's no more than a minor annoyance.