Note: this app is very specific. Only for those preparing for the TOEFL exam, a test for students wishing to study in the US.
You'd be surprised how many emails I get a week asking for apps to learn better English. Students from all around the world ask me to feature a great app to help them one day come to the US and study.
I've finally found a great one: iTooch TOEFL. Here are the stats on this one: tips on reading, speaking, and writing. 50 chapters with 1,508 questions. 135 images and 278 sound files to help immerse you in TOEFL-prep. And it drops from $4.99 to $0.99!