This page lists sites which have freely available online books. (Exception: Storytime Online) While a few sites include some recent books made available under Creative Commons or similar licenses, most of them are available because they are old enough for their copyright to have expired, (i.e. they are in the Public Domain).
When looking at the various collections below remember that works will often reflect the values, beliefs and prejudices of their time period. Be sure to take this into account when selecting works appropriate for your readers.