is a great source for audiobooks and all sorts of topics - classes, science, literature, nature, history, etc. Content is free and there's an iTunes U app for your iDevices.
For audiobooks, I like the Lit2Go
series of classic literature from the Univ of South Florida. You can download from either iTunes U or the website.
Currently, I'm listening to a lecture series from Stanford Univ on Hannibal (~13+ hrs).