Purdue University -- One of the best, most accessible sites. Self-help for students, handouts for teachers, advice on different kinds of writing tasks, excellent links, ESL and WAC sources. For those interested in writing center concerns, back issues of Writing Center Newsletter available.
Assistance for Speakers Whose First Language Is Not English
Ann Raimes ESL Tip Sheets -- An excellent source for understanding the features of ESL students' different native languages and seeing what kinds of errors the students will need to anticipate (sponsored by Houghton Mifflin).