One of the online tools that I have been using for formative assessment with my students is Formative. You can create questions for students to answer and then you can see exactly what the students do on your dashboard as they are completing the problems. You can create multiple choice and short answer questions, as well as white board problems. The most exciting tool on the site is being able to add a picture, such as graph paper and have students draw right on the picture. Students can even show all the work on the white board and submit it to me.
I have had opportunity to work with the Formative team to create some assessments. I and others took the Engage NY curriculum and rewrote the exit tickets into Formative. I am also working on rewriting the NYS regents algebra multiple choice exam questions into Formative. Check out the spreadsheet of teacher created formative assessments!!!