Reader (and musician) Matthew Becker would like one more capability rolled into his iPod: a metronome. "Why not," say I, "roll your own?"
Venture to Ambrosia Software and download a copy of the free WireTap -- the little audio doohickey that allows you to record any sound playing on your Mac.
Now fire up GarageBand or any music application that includes a metronome. Create a new piece with a desired metronome setting -- 120 for example. Start the metronome ticking and record a mean mess of audio. When you've got enough, stop recording. Those ticks will be saved as an AIFF file that you can then convert in iTunes to a format of your choosing. Save the file with a name such as "MM=120" and drop it into your iPod.
Repeat as necessary for different metronome tempi.
A just-for-fun-project: If you like, you can edit the start and stop times of your metronome files in order to make them loop evenly. With an even loop all you need do is record a couple of measures of click and then demand that your iPod repeat by selecting One from the Repeat command in the Settings screen.