Also an aggregator of open materials, but expansive beyond just textbooks. The default search results include fact sheets, graphics, syllabi, and a variety of other ancillaries, in addition to textbooks. Be sure to use narrowing options at left to hone your search.
Khan Academy offers practice exercises, instructional videos, and a personalized learning dashboard that empower learners to study at their own pace in and outside of the classroom. Includes math, science, computer programming, history, art history, economics, and more.