The Purdue Online Writing Lab website has excellent resources for ESL writers.
Many students find the explanations and exercises/self-access activities in these websites helpful:
Resources for Second-Language Tutors and Teachers
The TESOL association has a Second Language Writing Interest Section with a very good webpage with resources:
Faculty and tutors may find resources in these useful and relatively recent bibliographies:
Saenkhum, Tanita, and Paul Kei Matsuda. (May 2010). Second Language Writing and Writing Program Administration, WPA-CompPile Research Bibliographies, No. 4
Paul Matsuda, from Arizona State University, a key scholar in L2 writing, has listed some useful resources on his webpage.
For wider questions about language diversity, the Center for Applied Linguistics here in Washington DC has very useful digests online.