Whiteboard.chat
Whiteboard Chat is a free service that you can use to create collaborative whiteboards to use with your students. It is possible to use Whiteboard Chat without an email address which makes it quick and easy to get started.
There are two ways to use Whiteboard Chat. The first is to create one whiteboard that you share with all of your students. The second way to use Whiteboard Chat is the more interesting option. That option is to create individual whiteboards for each student to use that you can also observe.
To get started with Whiteboard Chat simply head to the site and click on the big "Start Drawing" button. Next you have the choice of "start collaborating" or "start teaching." The "start collaborating" option will launch a single whiteboard that you can invite your students to join. The "start teaching" option will launch an instructor whiteboard plus a grid of individual whiteboards that you can share with your students. When you use that option each student has his/her own whiteboard to draw on that you can also observe from your computer. In both cases you invite students to whiteboards through unique invitation URLs that you can post in Google Classroom, Microsoft Teams, or any other place that you typically post links for students.