I like the ChessU app for iOS. It's got ~100 courses for download, each $0.99 or $1.99. A course is typically 10-25 annotated games focused on some theme, and i

I like the ChessU app for iOS. It's got ~100 courses for download, each $0.99 or $1.99. A course is typically 10-25 annotated games focused on some theme, and includes lots of variations, expository text, and quizzes. Courses are available for basics, tactics, specific openings/defenses, and famous games/championships. I was always uncomfortable with D4 (queen's pawn) openings, so I went through the course on queen's pawn. It had 25 lessons covering everything (Slav, Albin, Benko...) and now I play D4 with confidence. It's made chess a lot more fun because I can play more varied games. (Disclaimer: I've no connection to the app or its author, except as a happy user.)