Here are the best mobile grammar and spelling checker apps, all of which are completely free and can be found on Google Play and the App Store.

Android apps English Grammar Spell Checker
This app is like a portable grammar dictionary that you can carry in your pocket. It corrects all your grammar and spelling mistakes as you type, helping you write comprehensive, clean, completely error-free sentences. It includes 20,000 words, but you can add more to the dictionary from every sentence you write.

The app also comes with a Text-to-Speech option, which is perfect if English is your second language and you want to learn proper pronunciation. The app can read aloud everything you type.