I've found a new web application that I think is "da bomb". It has two parts - one is a shareable "whiteboard", the other a web-accessible planning tool.
You can see a sample here. If you sign in for the planner, you can then create a whiteboard. I've made one to share with the other science teachers at my school. (see the example on the post above)
The whiteboards are a neat idea, particularly when you don't share common planning time, or are working with someone in a different time zone. (2nd example above)