Classicly has almost 1800 free novels in its Young Adults collection.  These are available for free download in PDF or Kindle (AZW).

Contra Costa County Library Lots of resources and e-books available. 

DigiLibraries has over 2,000 free e-books in their juvenile fiction category.

FeedBooks has around 200 free novels in its Original Books Juvenile & Young Adult genre as well as over 100 free novels in their Public domain (classic) Juvenile & Young Adult section.

Lit2Go has several classic free audiobooks for young adults.  All are available for free reading online without registration.

MagicKeys has 7 free books for young adults.  All are available for free reading online without registration.

Read.gov has 6 classic novels for teens available for free reading online or free download in PDF.