I have to be honest, I'm not a native English speaker, but this site helped me a lot with learning English - so I'm happy other people are recommending it! :)
As for a recommendation, I wanted to share mine with you as a way of thanking you for sharing ru.english-attack.com.
I recently found online a funny article about the English language (which made a lot of sense to me, as a non-native), and I'm pretty sure it will speak to your users as well.