PodReader is an open-source OS X application that takes electronic texts and "intelligently" parses them into a series of text blocks that can be read on Apple's iPod.
Still in the early stages of development, it is planned to rapidly mature into an application tailored for PDA-type functionality to include reading of various types of electronic media. Ports to Linux and Win32 are planned.
You can download the application from
here. Related to PodReader are two articles (
here and
here) which explain how to build an e-book reader for iPod.