English is important in so many ways that assessments are completely imperative. Even if it's just having the students make an illustration to show that they have mastered the vocabulary or a certain story that they are reading, it is still a way to tell where they are as far as how much they know about the topic, and what needs to be worked on.
In the classroom that I am observing at Ledford Senior High School, the teacher gives weekly vocabulary tests on Fridays. All of the English teachers I had in high school did the same. To me, this is a great tool of formative assessment. In fact, I believe that it is essential to give vocabulary tests. Studying English and Language Arts wouldn't do any good if the students didn't learn new words.
Some online formative assessment tools include ClassScape and Quia. These are great tools for high school students to use. High schoolers are usually eager to use technology in he classroom and on homework assignments, so these sites would open up new doors for them and make assessments a little more fun.