The websites below provide grammar quizzes that allow students to practice their skills while getting instant feedback and explanations. Just select your particular weakness from their lists of grammar terms and common errors, and you will be on your way to overcoming those pesky agreement problems and comma splices.
Commnet Grammar Quizzes: a massive list of sentence structure and punctuation exercises that also includes activities for building vocabulary, indentifying commonly confused words, and improving spelling
Penguin Common Errors Workbook: an online workbook that provides practice identifying and correcting errors in both sentence by sentence and full paragraph examples
Penguin Common ESL Errors Workbook: a list of grammar reviews and quizzes designed specifically for ESL students, including practice with verb forms and tenses, article use, and troublesome English words
Grammar Bytes Quizzes: an extensive and fun collection of grammar quizzes, complete with an applause track and imaginary prizes