Bibliotastic offers 11 free novels in their Teen & Youg Adult Category. These are contemporary works by authors who are generally unpublished and are unique to this site.
BooksShouldBeFree offers around 200 classic works in their Teen/Young Adult category. While primarily an audio book site, they also provide links to reading versions at other sites. Nice collection of classics.
ClovisNews offers about 100 free classic novels for free reading online (the download links all appear to be dead).
DigiLibraries offers 1348 free novels in their Juvenile Fiction category. This is a mix of classic and modern (unpublished authors) works. All the ones I checked were available for download in EPUB or for reading online. Some also offered PDF downloads. Thanks to Jonathan Malka for this suggestion.
DigitalBookIndex offers over 500 free classic novels in its various Juvenile & Young Adult Fiction categories. Main link is to the general category. They also have Boys and GirlsMystery, Science Fiction and Non-fiction (Teens) that offer differing works. Seems to be equally divided between PDF downloads and reading online, with some MS Reader. categories as well as
EPUB has close to 100 free classic novels in its Young Readers category, which they define as fiction annd non-fiction for teenagers. They also have a listing of free books in their Coming of Age category. All are available for free download in EPUB. There are links to other formats, including audio, but these appear to be at pay sites.
EPUBBud has about 70 free books in its Teen section. These are available for free download in EPUB. These appear to be a mix of classic and original and unpublished works. You can also create your own book to post there. Thanks to lala for this suggestion.
FictionPress has a huge listing of free books and short stories in its Young Adult section. These are works from unpublished authors. Most appear to be short stories, but there are some novels in there and there is sorting available for that. These are available for free reading online.
FreeOnlineNovels has around 200 free novels in its Young Adult category. These appear to be original books from unpublished authors. All are available for free reading online and some for free download in PDF.
FreeEbooksForYoungAdults is a site maintained by some authors at Smashwords (below) and features free eBooks for young adults in a blog like listing. Like all listings at Smashwords, these are available for free in a wide variety of viewing and download options. I do not know if this has different listings than the Smashwords' listing below.
GetFreeEBooks is a blog like site that has listings of free books from around the web. These are generally unique entries not found elsewhere. There are about 30 free books listed in their Young Adult category, though these may be time limited.
ManyBooks (Young Readers) 2443 free classic books available for online viewing or download in an extremely large variety of formats.
oBooko has just over 100 free novels in its Teen & Young Adulte category. Registration is required for free download in pdf. These are generally from self-published authors. There is also a section of short stories from younger writers.
Smashwords has close to 800 free book listings in its Young Adult or Teen category. these are available for free online viewing or free download in a variety of formats. These are generally unpublished works and unique to this site. The site does also offer books for purchase, but all the works in this list are available for free.