Footnote lets you record audio notes while you listed to audio books or podcasts, for example. Simple enough, right? Think about how something like this could be extended in so many other ways though.
Why limit yourself to audio products? What about e-books? Wouldn't it be nice to be able to record audio notes while you're reading that how-to guide? Then there's the textbook angle. Imagine something like this being available to students so they could "make a note" right in their e-textbook; the note is spoken, not written. Take it a step further and imagine the student being able to record the portion of their teacher's lecture that relates to this particular page or section of the book.
This functionality requires audio input, of course. Gee, there's yet another feature Amazon hasn't considered for their Kindle family. Doh! Apple is on top of it though as even their upcoming iPad has a microphone.
Although I think the Footnote app is pretty cool, I worry that it's a feature that would be extremely easy for all reader app developers and device makers to add to their core products. If they do, I hope they think of all the other features that could also be added. Social networking is an obvious one. Maybe you'd like to tweet a few of your notes. A more impressive feature is one that collects all your notes into a separate audio study guide, something you could listen to during the daily commute or even share with (or sell to) other students.
Even if you don't have an iPhone/Touch you owe it to yourself to check out the Audio Footnote website or watch their demo video that I've embedded below. If you come up with additional feature ideas I didn't think of please be sure to share them with the rest of us!