Grammarly is a free Firefox extension which checks your spelling and grammar on Facebook, Twitter, Gmail, in web forms and just about anywhere else you type online.

Once installed, the system works automatically. Start typing in a supported textbox, Grammarly displays its icon in the bottom right of the box to show it's working, and problem words or phrases are immediately underlined in red.

It's a familiar idea, but Grammarly goes a lot further than some of the competition. Context-sensitive checking means the system doesn't just complain about spelling (theyre), it also knows when words are used incorrectly (go over they're). And the grammar checker moves beyond the basics to tackle very complex errors and issues.

Fortunately, none of this complexity is visible to the user. If there are no mistakes, you'll see nothing beyond the Grammarly icon. If there are problem areas, they're underlined in red, and hovering your mouse over them displays a pop-up with an explanation and some alternatives. Click one of these suggestions to replace the original text, or tweak it manually, and carry on typing.

Sign up for a free Grammarly account and "you will start to receive weekly emails with personalized insights and performance stats". Doesn't appeal? No problem, it's not compulsory, the extension will carry on working regardless.