The Purdue University Online Writing Lab (OWL)offers an extensive collection of handouts and interactive quizzes.
Dave's ESL Cafe has several resources, including a list of idioms and their meanings, as well as dozens of interactive quizzes.
English Club provides English grammar lessons for ESL students, and also for consultants who may need a little brushing up.
ESL Resources is another site with resources ranging from grammar exercises to critical reading and writing exercises.
Pasadena City College Writing Center has many grammar handouts and some interactive quizzes.