CK-12 (https://www.ck12.org/teacher/) is a free open-source website that has FlexBooks, simulations, study guides, practice questions, and more!
Their mission is to “provide free access to open-source content and technology tools that empower students as well as teachers to enhance and experiment with different learning styles, resources, levels of competence, and circumstances.”
They provide translations in 6 different languages and their resources go from kindergarten to college-level Math and Sciences. Pictures, videos, hyperlinked vocabularies, and flashcards make it easier for students to visualize and access content. I’ve used their interactive simulations/games for online lessons and my students reacted quite positively. Labs and simulations can be done with their own devices and students can do the same activity even though they are not physically in the classroom together. It’s a great resource to try out!
Here’s an example of what their FlexBooks look like:
I have used CK-12 in my class, and found it to be a very useful resource, especially during remote learning. I actually signed up for it a few years ago and have watched it grow from just a basic workbook/worksheet type of site to a more interactive site with videos and simulations.
Sometimes I find it a bit difficult to navigate and organize exactly what I would like – it isn’t always as intuitive as I think it could be, but overall, it is a very helpful supplemental resource for the science and math classroom.