I like a lot of the ideas Jacob has posted here. I'm interested in OCR and/or audio recognition stuff, so the essay scanning, subtitles for conversations (+ translation), or the piano grader apps stood out for me. The best idea I had come up with so far was to use OCR to convert vocab lists at the end of textbook chapters or words in the glossary into flashcards that the user could study using their phone. This would be targeted towards middle school and high school students who have vocab quizzes. There seemed to be a lot of flashcard apps in the Android market, but none that I saw skipped the time-consuming part of having to enter in all the text for the flashcards yourself.