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

This Windows and Mac app allows you to use the program outside your web browser, either as a free-form word processor or through importing text via the clipboard ot by dragging compatible document types into the program window.

Once installed, the system works automatically. Once text has been imported or typed into its main window, Grammarly goes to work, with problem words or phrases 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.

You'll need a free Grammarly account to use the app, which covers you for critical issues. A Premium account (currently $29.95 a month, or $139.95 a year) unlocks over 400 further checks and features, with the promise of improving your work beyond all recognition. For most people, however, the basic checks should suffice.