A Writer's Reference by Diana Hacker

The online version of the sixth edition of this print resource has some great interactive tutorials and exercises for both ESL and non-ESL students. My favorite, for example, was an exercise targeting wordiness issues, which has students practice eliminating as many elements from a sentence as possible without altering the meaning. I always found this a difficult problem to help students with (in my non-writing courses), and I could see this as a way to give a targeted exercise to a student who needed it, and then have her tackle a rewrite (or the next paper) using what she learned from the experience. Most are available for free, but instructors can also adopt and have students purchase the text to unlock additional companion resources.