Twiddla is a free service that lets you hold online meetings. There is no setup or signup to use it. You can mark up websites, graphics and photos or use a blank virtual whiteboard. It works on any browser or operating system.
It's great for virtual meetings, conference calls, help sessions, study sessions, etc. for teachers, schools and students.
There are paid versions with more features. It's always free for education.
Here are some screenshots of it in action.