Are there any services or codebases available that provide a shared real time work space that could be embedded onto a site?

Are there any services or codebases available that provide a shared real time work space that could be embedded onto a site?

I'm trying to build an educational web-app and want to provide a shared work space such as notepad that both users can see/edit, or a notepad that both users can see and one user can edit.
Does anyone know if such a service exists as opensource code or paid service?