Cambridge Vocabulary in Use series - An excellent supplemental text, especially for the more academically inclined. The self-contained two page format allows students, teachers, and tutors to pick and choose materials.
Cambridge Grammar in Use series - This series is the only grammar series that I've ever felt comfortable using in the classroom. Again, the accessible, clear format with self-contained lessons allows both self-study and effective use as a supplemental text.
Writing Academic English, by Alice Oshima and Ann Hogue, provides practical techniques for students planning to attend community college or university. I think the latest version is the fourth. I've used it in several programs with considerable success.
In Focus: Strategies for Academic Writers
by Myra
Ann Shulman
Is my current choice for intermediate ESL students.
The clear, detailed exercises allow students to learn academic writing
by actually writing short, focused pieces.
Finally, for advanced ESL students and international graduate students,
I strongly recommend
Academic
Writing for Graduate Students,
2d ed.: Essential Tasks and Skills (Michigan Series in English for Academic & Professional
Purposes)
by John M. Swales and Christine A. Beer Feak
I teach
two courses using this textbook at USC, and students make clear, significant
progress by completing bite-sized writing exercises and analyzing short
journal readings. The teacher's guide, called Commentary for, also deserves
to be on your list.
Any Longman Dictionary
For whatever reason, Longman
dictionaries seem much more accessible and practical in their design
than other series. I’m particularly impressed with their Business Dictionary
for English Language Learners.