The blog Productive Strategies has compiled a number of free academic podcasts. Several more appear in the comments sections at the bottom.
Survey of Global History - Purdue
Introduction to International Relations - Purdue
Intro to American Politics - U Washington
Tools for the Information Age - U Hawaii
Understanding Computers and the Internet - Harvard
Intro to computers - UC Berkeley
Also, Princeton's University Channel has TONS of lectures in a variety of formats (e.g., streaming audio, video, mp3).
You may also want to try the online service offered by blinkx, which monitors over 15,000 podcasts and vlogs (video blogs) with a voice-recognition ap.
ADDENDUM: Mangolab has a nice tutorial on how to add RSS/XML podcast feeds to iTunes. Of course, for mp3 files, just right-click and save.
Tags: podcasts, teaching