Audacity® is free, open source software for recording and editing sounds:

Teachers can use this to record their voice to create a lesson for their students or they can use this in their classroom for students to create projects for example:

Overvoice on a commercials.

http://audacity.sourceforge.net/
Sample Podcast Lessons:
1) http://k12handhelds.com/podcastexamples.php
2) https://sites.google.com/site/pod2009site/podcasts---audio
3) http://allthingsaudio.wikispaces.com/Podcast+Examples
4) http://www.how-to-podcast-tutorial.com/17-audacity-tutorial.htm
5) http://www.learnoutloud.com/Content/Topic-Pages/Introduction-To-Podcasting/17