DuoBook - Easily switch between reading and listening to your favorite books.
LibriVox - free public domain audiobooks
Pocket Listen - Reading is hard. Listen to articles instead.
OverDrive Media Console - Download eBooks and audiobooks from your library directly to your iPhone, iPad, and iPod Touch!
Audiobook Player - An audiobook player using an embedded Microsoft mediaplayer
Listen Audiobook Player - Feature rich and easy to use player for your audio books.