This site has many simulations helpful in understanding various computer science topics taught in high school.
The index has a list of all the topics taught on this site.
Some favorites:
An online interactive resource for high school students learning about computer science.
Open source
This project is completely open source and available on GitHub.
CS Unplugged is a collection of free teaching material that teaches Computer Science through engaging games and puzzles that use cards, string, crayons and lots of running around.
Printables
This page diplays a complete list of all available printables. If a lesson uses a printable, the lesson will contain a direct link to the printable with a description on how to use it.
A computer science curriculum that makes coding easy to teach and fun to learn. No CS experience required CS First empowers every teacher to teach computer science with tools and resources that are free of charge. Fun hands-on learning Students learn through video tutorials and block-based coding in Scratch.Always free of charge CS First is totally free of charge — any number of students, all materials, as many lessons as you want.