There are probably more sites than simply ClassMarker that involve quizzes, however this is the one in which I have had experience with during my e-learning journey. This particular site provides teachers with the opportunity to create online quizzes for students to complete that will be marked by the website, providing both student and teacher with immediate results. I believe this would be a really good site for teachers to incorporate into their classrooms practices as it encourages students to activate their prior knowledge and to use higher order thinking to answer the questions within the quiz.
I personally don't feel that this is something I will use in my classroom a great deal as it does not cater for all learners' needs. I feel that quizzes such as these do not provide a very accurate means of testing and that gauging a students knowledge and ability is not the best way of finding out what it is they know and what needs to be taught in more detail.
Keirsley and Schneiderman's Engagement Theory applies to this tool as students would be engaged and enthused by the fact that they are receiving on the spot feedback. They would more than likely see it as a type of competition between peers.