HEXAM is WordPress plugin for providing online examinations, quizzes. You can create online exam-tests or quizzes, provide competitions between users in your site very quickly. It is available here: http://wordpress.org/extend/plugins/hexam/.

Description from readme.txt:
By using an HEXAM PLUGIN (examination system) you can provide online exams,quizzes in your web site. It is very easy, you can create a lot of tests in your site and place it in
posts or pages. You can choose how many questions each quiz may contain, you can also set number of answers for each question. Users can send their results after doing test. You can also view
how many points each user has scored and you can see and post users’ ranking table in wp.