Wikibooks has more than 2,900 non-fiction books online, categorized by subject area.  The books are readable online in wiki format.  You can browse the titles by subject, completion status, or reading level.  A special Wikijunior section includes titles for K-5 students.

Baen Books offers dozens of science fiction and fantasy titles for free for download in a variety of electronic formats. eReader formats supported include Mobi/Palm/Kindle, Ebookwise/Rocket, ePub/Nook/Stanza, Microsoft Reader, Sony Digital Reader, and RTF.

The site has indexed more than 20 million titles, with the goal of having a listing for every book ever published.   The listings include 1.7 million books in the public domain.  You can download many of these in a variety of formats – PDF, ePub, MOBI, DAISY, DjVu and ASCII text.  Others can be “borrowed” by providing an email address for checkout.  Note that some titles are still copyright protected and have links to sites for purchase.

Read.gov provides free public domain ebooks online for kids, teens and adults.  Their online ereader is easy to use and includes full color illustrations, when available. You can also watch webcasts of well-known authors, who talk about their books and how they write.  And the site offers contests for K-12 students, along with teaching resources.

This is a collection of children’s books from around the world in dozens of languages. The mission of the ICDL Foundation is to help children appreciate diverse cultures, languages and ideas through literature. Search for books by continent, language, author and more.