Proofreading: Spelling and Writing is a really nice series of many, many exercises where students have to correct paragraphs. It’s divided by levels of difficulty.
I’m adding it to The Best Online Interactive Exercises For Writing That Are Not Related To Literary Analysis (It’s a long story about why that list has that title — you can read the explanation in the post).
Thanks to the Langwitches blog for the tip.
You find a lot of great stuff, but I have to say that this website is one of the best. I highly recommend it to English teachers.