I want to share with you plugins which i use in most of my WP powered sites and hope they will be useful for you. In comments section you can continue the list and write your favourite ones.
1. HEXAMHEXAM is Wordpress plugin which lets you to provide online quizzes, mini-exams. Number of questions of each quizz and number of answers of each question you can set yourself. The plugin supports full html,css and has not rich default css, so you are free in building very different interface.