Insight Meditation Center
This is the best podcast or audio series I've found. It's a complete and thorough meditation program. It provides fundamentals, theory, best practices and quality guided meditations. The Insight Meditation Center is a Buddhist Center teaching Vipassana meditation. If you're new to meditation, I'd recommend starting off with the Meditation Oasis or Meditation Society of Australia podcast and then moving on to this series. It might be a bit too much if you're brand new.
Podcast: Meditation Oasis
A musical and relaxing series of guided meditations, the Meditation Oasis is a great place for beginners or people who want to use meditation to get rid of anxiety. I don't think you'll reach any higher states of consciousness using this program, but you'll feel soothed and content.
The Meditation Society of Australia
Podcast: Learn to Meditate - Meditation Podcast
A lecture series with guided meditations, the Meditation Society of Australia emphasizes positivity and love. The guided meditations require lots of visualizations, so if you don't have an active imagination I wouldn't recommend it. The lectures were slightly too "feel-goody" for me, but they did include plenty of informative and worthwhile points. I recommend the series for beginners.
Podcast: Motivational Power - How to Change your life and create your future
I cannot recommend this podcast/audio series. It is full of unrealistic promises, wild claims and unhelpful practices. For example, the author claims if you accurately perform one guided meditation, you'll become rich over night. However, he includes the caveat, it takes years of practice in order to correctly perform this meditation. He keeps his listeners in a catch 22; keep listening and learn unbelievable secrets to power and wealth, but if it isn't working for you it's because you aren't doing it right. Nowhere does he provide ways to correctly meditate to learn and grow.