Books Should Be Free Teen/Young Adult listing of free public domain audio books and ebooks.
Lit2Go a great collection of free literature that also includes a PDF of each selection.
Open Culture offers many classics in a variety of audio formats.
LearnOutLoud free audio library. Other links on the site offer retail audio books.
DigitalBook has is very easy to navigate and searches by more genres than many of the other sites. Check out genres like Teen, Chick Lit, Comedy, etc.