This is the most popular. They have extensions for most browsers and even an editor. Once the extension is installed, you'll notice it on any online editor. You'll recognize their green G icon usually down in the right-hand corner of your document. Like a spell checker, it will scan your document and underline anything it thinks needs correction.

Just run your mouse over it and it will suggest a correction.

As I pointed out, you can download their Windows app. A basic editor. The free version limits how many corrections it will make and you need to set up an account even with the free one.

The Grammarly editor allows you to import a document and to save it as a .docx after.

This is an online option that does basically the same thing. It does not have an extension, and no account required. Also, it only has a copy and paste function to get your documents checked. You can pick your language either, American or British English and hit the Lookup button.

If you're a real stickler, hit the Deep Lookup button. It sends you to Grammarly. Otherwise, it works the same way. It'll highlight potential errors and make suggestions. Copy and pasting seem to be the only way to get it back after.