This website is a resource center for teaching and learning about American English and American culture developed by the Office of English Language Programs in Washington, D.C.
Duolingo is a popular mobile app that gamifies learning languages for users. Free version available. Please note that the premium version requires a paid subscription.
The creator of Grammar Girl is Mignon Fogerty, who offers tips to help you remember many rules of English Grammar. Check out her blog posts and podcasts at https://www.quickanddirtytips.com/grammar-girl.