In my Algebra 1 classes I had prepared a problem set wherein students had to arrange numbers from least to greatest. This specific task has many levels of complexity as evinced by my creation of varying difficulties for the students. For those who needed more work towards the beginning, they would focus on the first two levels and practice to get the answers correct. For those speedier students, they could fly ahead to levels 3, 4, and 5.
I have also used this system to take quick polls from the class on a variety of issues and graphically display the responses - something quite applicable to a math classroom.
Formative assessment is a tough thing to perform and I'm happy to have my hands on a tool that can ensure I push myself to use data to make a difference.