The ESLQuizCenter (by Dave Sperling) Quizzes on grammar, slang, confusing words, punctuation, reading comprehension, etc.
Activities for ESL students web site (by many teachers) Bilingual quizzes (Chinese-English, Japanese-English, Spanish-English, Korean-English, Russian-English, etc.), general grammar, and vocabulary quizzes for different levels of proficiency, etc.
ESL Blue(s) (by Eifion Pritchard) A general grammar site for ESL students, with a special focus on verbs.
English Quizzes (by S. Kathleen Kitao and Kenji Kitao) A site on grammar and vocabulary. Also general quizzes on proficiency in English and reading comprehension exercises.