How do you keep track of ideas that pop into your head?

I am curious how others deal with the problem of recording and tracking ideas that pop into your head. In particular, what electronic or software tools do you use to organize ideas and random information that comes up in your professional life, the information that does not fit cleanly into a PDA calendar or address book.

Half of my random information stays only in my head, some gets written down on index cards, some gets entered into text files in "My Documents", some events get recorded in my PDA calendar, and a lot of stuff is stored away in email. Sometimes ideas end up on email lists or in blog posts such as this as well.

That may be fine for recording, but without any good organizing the value of any idea falls off a proverbial cliff.

As they say, there has to be a better way.

To put the question more simply, when an idea pops into your head, what do you typically do with it? Especially when those ideas occur when walking, driving, eating, showering, etc., rather than when you are sitting at your desk and computer.

-- Jack Krupansky