Duolingo uses the student's native language to teach English. It incorporates speaking, listening, reading and writing. Great for all levels, but especially Levels 1 and 2.
Busuu has many different activities to practice English, including listening, speaking, reading and writing. There is also a part where you can interact with native English speakers. Great for all levels!
Drops is a language learning app that does uses only pictures and audio to teach English. Good for all levels.
Quiz Your English
Play this game to practice English with students from around the world! Compete to earn points while practicing the English you have learned in class. Good for levels 3&4.