Free audio books or podcasts are usually in mp3 format - so you need an mp3 player to be able to play them. You can also sync these with your iPod using iTunes
Librivox - Volunteers read out of copyright books as Talking Books in mp3 format.
Literal Systems - also read by volunteers, but more recent readings of the Talking Books ar made by paid professional talking book readers. All Literal Systems talking books are free.
Internet Archive (Audio) - Many thousands of recordings, news recordings, presidents talking, old time radio shows, archived podcasts, book reviews, and more.
BBC Podcasts - listen to your favorite BBC radio program, documentaries, talking heads, book reviews, and more, in mp3 format.
ABC Radio Australia podcasts - all your favourite ABC radio shows on podcast, book readings, documentaries, talking about a wide range of subjects on Radio National, and more
Podcast Alley - the lasted on popular podcasts, from radio shows, episodes of talking books,
A sample of some of the (not free) Audio Books available at The House of Oojah - we have mp3 Audio Books click here - instructions on converting Audio Books on CD to mp3 are here, instructions on using iTunes for converting Audio Books on CD to use on an iPOD is here