I've found this pretty helpful when practising exam questions that would normally need pen and paper to work out, and when pointing out parts of screenshots to other people.

Some features that I'd like to suggest:
- A ruler and protractor tool to measure parts of a web page
- Keyboard shortcuts (e.g esc to stop the extension)

actually pretty awesome, only a few features that would be very useful :
- clean all button : to clear the page
- text font management
- visible border for texts (when hovering over them)
- adding keyboard hotkeys for faster notes