
“100% Free, Personalized Learning for Every Student”. Sounds like a hefty claim, I know, but these folks at cK-12 seem to know what they’re doing! cK-12 is an amazing resource for Math and Science so it feel it eminently appropriate for sharing here 🙂
As you can see below, cK-12 offers up free, online textbooks for most major math and science topics up to, and sometimes above, high school level:

Each topics tends to have a “Concepts” section, an accompanying “FlexBook Textbook” (which is essentially a build-your-own-textbook tool for students and teachers), and PLIX – Play, Learn, Interact and Xplore. PLIX is very cool, and does what it says on the box, so to speak; allows students to learn through exploration and interaction with topics in an extremely visual way.
Take Trigonometry for example. A student could explore a PLIX related to the topic they’re struggling with, such as inverse trig functions, and it will not only visualize the situation but also give them small topic-related practice problems. It’s quite a powerful tool. See below for a little taste:

There are a massive number of these PLIX tools for each subject. Essentially it’s just a matter of finding what you’re looking for, vetting it for content and quality as well as how appropriate it is for use with your particular students, and away you go.

All in all cK-12 is an extremely useful resources offering up a number of tools absolutely free, with the only hurdle being that you need to create an account to take full advantage of its features. I recommend you give it a try (what ya got to lose, right?) and I hope you find it as useful as I do. Enjoy!
Scott