The free app Practice English Grammar from Cleverlize is among the most polished, and is easy to use for improving your grammar skills. It’s available for both iOS and Android and covers the whole gamut of grammatical details from conjunctions through tenses to using the passive voice.
Its main interface is a pleasing graphical display of your progress in each of the various modules. Tapping on one of these modules takes you to a section where you can see the grammar lessons in the form of flashcards, and then a section where you can test your knowledge in an interactive quiz.