Project Gutenberg also gives you EPUB downloads for any of their ebooks. (Though some older ebooks are transcribed/processed as text-only, so the HTML/EPUB is autogenerated and loses the original structure.)
It's not just "cleaned and make to look nicer". StandardEbooks ebooks contain editorial changes from the original text. They're tagged in the changelog, but they're mixed with technical edits so there's no easy way to revert them. I'd rather read the Project Gutenberg versions.