This site has over 1,000 simple vocabulary activities for ESL learners of all levels. All the quizzes are easy to use and have a drop-down box that shows you the correct answer. Activities are divided by Ability Level (Easy, Easy to Medium, Medium to Difficult and Difficult) and then by topic. This is a great site for a quick topic refresh, or to practice a particular vocabulary area in more detail.
Self-Study Vocabulary Quizzes