Wikisource
Contains full-text html versions of many French literary works, of all genres, from the Middle Ages through the 19th century, which are now in the public domain

Poésie Française at Webnet
Contains numerous poems in html format by French authors from the Middle Ages through the 19th Century, which are now in the public domain

Google Books
Contains full-text books of many old works up through the early 20th century that are now in the public domain, and downloadable in pdf format
It's usually best to do an "Advanced Search" and click on "Full View Only"