Many people download free royalty free ebooks from Project Gutenberg. It is the largest site in the world for this type of content and offers downloads in EPUB and MOBI for Kindles.  One of the best alternatives is Standard Ebooks who reformats ebooks from PG and edits them, adjusts the typesetting, add better cover art and metadata.

The Standard Ebooks project is a volunteer driven, not-for-profit effort to produce a collection of high quality, carefully formatted, accessible, open source, and free public domain ebooks that meet or exceed the quality of commercially produced ebooks. The text and cover art in our ebooks is already believed to be in the public domain, and Standard Ebook dedicates its own work to the public domain, thus releasing whole ebooks files themselves into the public domain.

The Standard Ebooks project differs from Internet Archive and Project Gutenberg. They focus on typesetting, cleaned of ancient and irrelevant ephemera, take full advantage of modern e-reading technology, are formatted according to a detailed style guide, and that are each held to a standard of quality and internal consistency. Standard Ebooks include carefully chosen cover art based on public domain artwork, and are presented in an attractive way on your ebookshelf. For technically-inclined readers, Standard Ebooks conform to a rigorous coding style, are completely open source, and are hosted on GitHub, so anyone can contribute corrections or improvements easily and directly without having to deal with baroque forums or opaque processes.

Ebooks from their catalog can be downloaded as EPUB, AZW3 (for Kindles), KEPUB (for Kobo) and EPUB 3. You can visit the site from your e-reader, using the internet browser to download content directly to your e-reader, without the need of using a PC.

Michael Kozlowski is the Editor in Chief of Good e-Reader. He has been writing about audiobooks and e-readers for the past ten years. His articles have been picked up by major and local news sources and websites such as the CBC, CNET, Engadget, Huffington Post and the New York Times.