Another one of the CK12.org tools I mentioned.
There is no doubt that if you are looking at differentiation, flipping your classroom or just another classroom resource that this is a great site. BrainGenie offers a number of skill organized exercises. It also has a great tracking system and provides a video tutorial for each skill. Skills are organized via goals. In my example, I am going to create a student goal of solving equations using integers. After uploading your student roster you then can assign the goal to your class or a specific group of students.
In addition to Braingenie I create small formative assessments, using Examview and Moodle, that I use to further differentiate. I made a short screencast of what I have learned on using BrainGenie my classroom.