Wikisource: downloadable classics & other public domain books (assembled by volunteers just like Wikipedia). Not pdf.
Ebooks-gratuits: Search by keyword or category.  Format is pdf.
Ebooksgratuits: Simply select the author (e.g. Grimm) or the catégorie (Nouvelles-Contes = short stories & tales). Several ereader formats available, including pdf.
Livrespourtous: Searchable by category & downloadable in several formats.  5000 ebooks available.
Livregratis: Searchable by category.
Gutenberg: The original HUGE public domain ebook project in English, with titles in French and many other languages. Browse French books or search. 

Tumblebooks
Watch animated kids' picture books online with narration, music, and sound. There are also language-learning games & puzzles. Book language options include English, French, Spanish, or Russian. TumbleBooks requires Flash.