CK-12 is an excellent Open Educational Resource (OER) that is free and easy to use. Digital resources include PLIX interactives and simulations, adaptive practices, study guides, and FlexBook digital textbooks. CK-12 FlexBooks are complete online platforms for assembling, authoring, and distributing standards-aligned, interactive, multi-modal educational content.
Ignite Creativity in your Chromebook Classroom: 60+ Tools and Lessons to Spark Creativity is an interactive book with images, videos, and audio that introduces educators to new ways of teaching and creating through technology.
A coalition of education organizations led by ISTE has curated free tools, strategies, tips and best practices for teaching online.