On the bus, in the car, on a plane or anywhere else, JapanesePod101 is great for studying Japanese! It has many audio lessons and resources to help you learn Japanese for all levels of skill. Each lesson comes with a pdf that covers what has gone on in the audio lesson, in great detail and it also has other stuff too like a kanji sheet. It also has a paid option where you get a whole bunch of new useful resources but in my opinion the free version is good enough and will serve you as an excellent tool in your journey to learning Japanese!

http://www.japanesepod101.com