Gallica: the digital library of the Bibliothèque Nationale de France.
The European Library: access to the resources of the 48 national libraries of Europe. Search its French holdings:
ARTFL Project: "nearly 3,000 texts, ranging from classic works of French literature to various kinds of non-fiction prose and technical writing. The 18th, 19th and 20th centuries are about equally represented, with a smaller selection of 17th century texts as well as some medieval and Renaissance texts. Genres include novels, verse, theater, journalism, essays, correspondence, and treatises. Subjects include literary criticism, biology, history, economics, and philosophy."
History of France: primary sources: full text of documents for French history from antiquity to the present.
Culture.fr: "le portail de la culture". Its collections contain millions of documents and images from various cultural institutions, all gathered in one website.
Europeana: a portal to the online collections of the most important cultural institutions of Europe, including the Bibliothèque Nationale de France, the Cité de la Musique, and some of the most important museums such as the Louvre and Musée d'Orsay.
Manioc: "bibliothèque numérique Caraïbe, Amazonie, plateau des Guyanes". A collection of text and multi-media resources, both primary and secondary, on the history of this region.
Wikisource: a free collection of French books in the public domain.