I'm using Pimsleur to bootstrap some phrases and simple grammar. IMHO this is a great way to get started on speaking the language and practicing pronunciation, at least if you are self-studying.

I bought this because it was cheap and it covers a variety of levels: http://www.amazon.com/gp/product/0307972232/ref=oh_details_o.... I am not sure how useful it is for me right now, but I expect that over time it will be helpful for small collections of vocab. (It's also pretty cheap!)

I wanted a book that dealt with grammar, so I bought Basic Korean: http://www.amazon.com/Basic-Korean-Grammar-Workbook-Workbook.... It has been well worth it in my opinion.

Quizlet is an invaluable resource for this: quizlet.com. They have text to speech so you can hear pronunciation for arbitrary words, and lots of people have made sets for different collections of vocab. It's not just flashcards, as it offers you a few ways to test your knowledge. And there are apps for Android and iOS.

If anyone else has found useful mobile apps, I'd love to hear it. For the most part I haven't found anything that I thought was truly great, so for the most part I just listen & practice the Pimsleur lessons when I'm in transit.

Hope this helps!