CK-12 is a massive website that includes a searchable library of K-Adult resources for Social Studies, English, and adult studies, but with a particular emphasis on Math and Science. A big part of CK-12 are their "FlexBooks" which are customizable text books, many of which include interactive content. It also has adaptive practice questions for Math and Science, Simulations, and pre-made Study Guides for High School level Math and Science.