Google Jamboard: Free online collaborative whiteboard. You can draw right on the whiteboard and students can show their work live on the whiteboard.
Here is a picture of a question we used the Google Jamboard for in a workshop with students. You can add sticky notes, pictures, text, and write live. You can Share Screen during your lecture and show the Jamboard.
Desmos: Free online graphing calculator that includes digital activities for students. Teachers can create accounts to assign student activities.
Free Math App: A platform where teachers can assign, review, and leave feedback on student work.
Geogebra: Online tools for graphing and geometry. Classroom resources and activities are free.
Teaching Math Online by Matthew Cheung. This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.