Language Tool

I discovered this tool because it is the system used by Ulysses for spell and grammar checking.

Not only does it allow writing in multiple languages, but it also integrates with the most common tools such as Word, Pages, Mail, Slack, Notes, and even Messages on Mac. It is also available for Windows and as an extension for most browsers.

The best part is that it has a free version that works for basic tasks like correcting this short text you are reading.