Practice makes perfect with the Wolfram Alpha Problem Generator. They have a huge database of problem types for you to springboard off. Additionally, their partner site, Wolfram Demonstrations offers 12,000+ interactive applications for education, research, recreation and more!
Many of the scientific simulations on the PhET site (from the University of Colorado, Boulder) are now able to be utilized on Chromebooks. They are no longer flash-based simulations.
PhET offers simulations from elementary level through to university.
Help Teaching is a website where teachers can create printables for their classroom. The free version will allow you to make a test or quiz of up to 10 questions. There is content for all subjects and all grades on this site.
The CK-12 Foundation is a California-based non-profit organization whose stated mission is to reduce the cost of, and increase access to, K-12 education in the United States and abroad. CK-12 provides free and customizable K-12 open educational resources aligned to state curriculum standards. The resource they have assembled online is phenomenal.
Their service comes up anytime one googles “online textbook”, and as such, teachers are likely to find some excellent content to overhaul or extend exsiting assignments and textbooks already being used.