Apple's iPod classic allows to store and view text notes. Enabling iPod's disk use, you can simply put them as txt file in the Notes folder.
The problem is that I like to store them on my Mac, and just sync the corresponding iPod folder whenever I sync it.
The simplest free solution I found is to use VoodooPad Lite.

VoodooPad allows to edit hypertextual notes and to save them to the iPod. To automatic sync, but anyway useful.