Audio
With the exception of iTunes and other music services, audio is probably the most underutilized asset on the web.
ITunes itself is underutilized by most marketers. Think of the Video and YouTube for comparison of what audio could or should be.
Podcasts: Think of it as Internet radio. Certainly, if you do any telephone webinars, at the very least you should make them into MP3s and have them available to your audience.
If you're interested in copywriting is a great site that uses audio correctly. You can go there and listen to a live two-hour phone session with greats like Gary Halbert and John Carlton, two out of 3 of my favorite copyrighters. Got to http://www.michelfortin.com
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