Grammarly is a Chrome extension that spell and grammar checks ALL your web typing - on Facebook, Twitter, Email - wherever you type, Grammarly jumps in to help!  A little G appears at the bottom of the page or text box where you are typing.  A red underline appears when you spelled incorrectly and a green line appears with grammatical corrections.  In addition, Grammarly keeps track of your errors, so you can track your improvement.