These are the tests that I use in grading my students. First, the test requires no paper, eliminating the battle for the copy machine. There is always the option of printing the test, if needed. Second, the student can take the test when and where appropriate. Even taking the test at home is an option if the student has access to the Internet.
Students can wait until they think they have mastered the material before being tested on it.
My students can send their results to me via e-mail on a form that I created, attaching the test as verification when necessary. Third, the test is a valuable teaching tool. These on-line tests provide both. They provide immediate feedback, requiring the student to eventually get the correct answer before moving on. This provides a huge advantage for the teacher and students.