Quizster is an interesting addition to the formative assessment game. Students can complete work on paper or in whatever app they want. They then submit their work to be graded to the teacher. The teach can mark up the document, give it a grade, and hand it back to the student.
The free tier allows a teacher to use one class with unlimited students. For more classes, Quizster has a $3 a month level for 3 classes, and a $5 a month level for unlimited number of classes.