Rewordify (web site), enter text or a web site address to intelligently simplify challenging words to facilitate increased comprehension, also includes tools to teach difficult vocabulary from the text selection or web site
PrintFriendly (web site), make any web site into a print-friendly PDF and selectively remove distracting content (ads, navigation links, etc.) to make information easier to read for students
Postlight Reader extension for Chrome, removes ads and clutter from web sites, can share files via email, Twitter, or Facebook
BeeLine Reader (Chrome extension), adds a color gradient to text to help your eyes track from the end of one line to the beginning of the next line
Grammarly (Chrome extension), contextual spell checker, grammar checker, and tone checker
WriQ add-on for Chrome, the free version automatically scores spelling, punctuation, and grammar
Quill (Chrome app) has free writing and grammar activities for students, includes a brief diagnostic to help create an individualized learning plan (Disclaimer: I haven't tried this yet, but thought it looked interesting)