I've tried a number of "whiteboard" products and I have to admit that Dabbleboard is easy and intuitive to use. While is doesn't have every single feature (ie resizable text), you can manipulate objects quickly, draw free-hand, insert text, add colour and insert images. You can download your production as a png file or sign up and keep it in your account. Dabbleboard also allows you to create a library of shapes you use often, so you don't have to start from scratch each time. And if you want to illustrate a point quickly, DB lets you draw without having an account: just click on the "Get Started" button and choose the new/or clear icon in the upper lefthand corner of the draw window.
Here's a sample:
