When I arrived to USA I used to listen to NPR podcast because I wanted to improve my English. I just wanted to listen to English native speaker talking about different subjects. I've never thought that pod-casting can be used in ESL classroom but after exploring English as a Second Language Podcast I see many ways it could serve students to practice their listening skills.
For example, this podcast http://www.eslpod.com/website/show_podcast.php?issue_id=9605140 is a dialogue between a waitress and a customer in a restaurant. It is a source of not only vocabulary used in a context but also provides an opportunity for students to listen a natural conversation in English by English native speakers. Additionally, the teacher could design a handouts based on the dialog with questions and blanks to fill in for explicit or implicit listening.
In fact, podcast could bee extremely helpful if ESL teacher is not a native speaker of English, thus the learners would became expose to a authentic pronunciation and language usage.