While not particularly user-friendly creation-wise, it is pretty easy to pick up on what you can do. It has math topics: integers, fractions, concepts, and geometry. Within each, you can choose specific concepts, level of difficulty, and the number of problems. Then, simply send students to the URL, and they can take a fill-in-the blank quiz and see results immediately.
This site seeks for teachers to enroll in its courses, but this is
a page of interactives that are free to use for educational purposes.
It also usually has "warm ups" for learning about the applet and "sample
activities."
Though this article is more than ten years old, the story of a Mars orbiter crashing because of forgetting to convert between English and metric units is a great example to use in class.