If you are looking for a free whiteboard give Mathlearning Center's whiteboard app a chance.
When you open the site you are presented with what you see below in the embedded picture. You will see all of your tools for writing, typing, drawing all on the right side of the site. On the bottom it allows you to hide portions, share the screen, as well as join a specific link.
Below that you will see examples that Mathlearningcenter has shared.