When teachers know their students well, they can build strong connections that lead to better learning. Knowing students' interests, strengths, and weaknesses help teachers tailor learning experiences for their students. Formative assessment is how teachers collect information about what students know, don't know, and want to learn. Formative assessment takes many forms, including exit tickets, discussions, games, and quizzes. These kinds of informal assessments can also help teachers get to know their students as learners and as people.
There is a very wide variety of digital formative assessment tools that can be used for free (often charging for extra features). I've written a little about 15 of them below. Most of these tools work with any web browser, so they are great for laptops, computer labs, iPads, Chromebooks, tablets, and smartphones.