Authonomy has 70 free ebooks under a search for Young Adult. All are available for free reading online without registration.
BookRix has almost 1000 free ebooks in its Juvenile fiction category. These are available for free download in EPUB or for free reading online.
Booksie has over 13,000 works in its young adult category. These range from short stories to novels and are unique works of unpublished authors. All are available for free reading online or free download in PDF (press "Generate eBook" button).
Classic Reader has almost 1800 free novels in its Young Adults collection. These are available for free download in PDF or Kindle (AZW).
ClovisNews offers about 100 free classic novels for free reading online (the download links all appear to be dead).
DigiLibraries has over 2,000 free ebooks in their juvenile fiction category as well as over 200 works in their Juvenile non-Fiction category and 27 free ebooks in their Juvenile Horror & Ghost Stories category. Most are available for free reading online as well as free download in PDF, MOBI and EPUB.
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.
GoodReads has a listing of 95 free young adult ebooks. All are available for download at their site.
MagicKeys has 7 free books for young adults. All are available for free reading online without registration.
ManyBooks (Young Readers) 2443 free classic books available for online viewing or download in an extremely large variety of formats.
PublicBookShelf has 5 free novels for Teens for free reading online. 2 are classics and 3 are newer and unique works.
Read.gov has 6 classic novels for teens available for free reading online or free download in PDF.