Writing for English Language Learners

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.
Pasadena City College Writing Center has many grammar handouts and some interactive quizzes.