GRAMMARLY is a FREE Browser extension for Firefox, Safari & Chrome that will check your Grammar and Spelling on almost all of your Social Media sites. I have been using it for the last 3 years and I love it! Check out this super short 1.20 minute video to see how helpful it can be! And check below the video for a link to the list of the most common Grammatical Errors!
Grammarly also has a FREE Microsoft WORD plug-in available here.