TalkToMeInKorean (TTMIK) provides free audio lesson and pdf for Korean learners from beginner to advanced level. Currently there are 8 levels. The lessons are explained in great details and very easy to follow even if you never learn Korean before.
Visit TTMIK's website: http://www.talktomeinkorean.com/
Their second website: http://harukorean.com/ provides more options for passionate learners who want to write sentences in Korean and have it checked and corrected by a native speaker. There's a $5.99/month membership for this service