Biguniverse.com Fiction and non-fiction picture books for elementary and early middle school students

Gutenberg.org  Literary works for all ages in public domain.

Tumblebooks  Online collection of animated, talking story books which teach young children the joys of reading in a format they'll love.  It even has classics, books for Beginning Readers, and books in Spanish. 
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These are just a few of the many resources you can find online.  Have another favorite?  Let us know what it is, and we'll add it to the list.