Conceptboard: This spring I went searching for a tool that would let me draw on a computer and show up on student iPads at the same time. I found Conceptboard, which is a collaborative online whiteboard originally made for the business world. You can draw lines, shapes, and text on an interactive whiteboard, and share the whiteboard with others.
When you share the board with others, you get a public URL in an email, which you can then share with your entire class. You are limited by whiteboard space in the free version, but you can create, save, and share these whiteboards to anyone with the internet.
Let's play with this board today: http://conceptboard.com/board/noxk-864g-166c-d1oz-m8yi