The TESOL association has a Second Language Writing Interest Section with a very good webpage with resources:
The Purdue Online Writing Lab website has excellent resources for tutoring ESL writers and more.
Many ESL instructors and their ESL students/tutees find the explanations and exercises/self-access activities in these websites helpful:
A useful and relatively recent bibliography:
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.