I guinea pigged my students with Curriculet.com a week ago, and I absolutely love the results. The website allows teachers to take texts and embed questions, annotations, and videos into it. Teachers can then track students performance and check their mastery. It’s a great flipped-classroom website, as this allows teachers to look at a glance whether students understand the material and then move onto more critical thinking activities during class.
In addition to adding your own texts, you can also use texts and “curriculets” from other teachers with questions already embedded. This site is still young and growing, but what a great resource it has the potential to become for all curricular areas.