Thank you for asking this question addressing what appears to be a sadly widespread problem in academia. I'm an international student (from India in US) myself, and it's been a great boon for me to be able to communicate fluently in English.
Particularly re: international students, our Office of International Students organizes (spoken) English classes, which are free for all international students and scholars. In addition, at Rice we used to have a group of grad students get together for lunch on Fridays and converse in English (this was a registered student organization called 'English Corner', and they had funding from various sources for the lunches). I realize that you were referring to resources for writing as opposed to speaking, but I believe that particularly for non-native speakers confidence in speaking can translate to confidence in writing. In terms of the actual writing process, there are some online resources, such as Englishforums
As for resources for a more general audience, in certain departments around here I've heard of a thesis-writing class that grad students are required to take for credit. Our Office of Graduate & Postdoctoral Studies has recently been trying to put together some professional development workshops and courses, which often focus on the 'communication' aspect of academia. Another possibility is that the beginning courses in the doctoral program (1st and 2nd year classes) could be made to have a strong (or at least non-trivial) writing component. For example, one might require students to write an expository term paper or something along those lines.
I realize that roughly everything I've mentioned above has to do with resources available at my university, so perhaps my answer consists of 'Here are some resources that might already exist at your university, or might be put into place there'.