For beginners

Assimil French: https://www.amazon.com/Assimil-French-Pack-English-speakers/dp/2700518136/
-- The book + CD pack that I used to start learning French. Well worth the price.

French In Action: https://www.learner.org/resources/series83.html
-- A video series teaching French by “immersion” (no translation) from the very basics to intermediate level.

Duolingo French: https://www.duolingo.com/course/fr/en/Learn-French-Online
-- French learning site / mobile app.

Italki: https://www.italki.com/home
-- Find native speaker teachers for one on one online tutoring. Rates for most French teachers range from $15 to $40 an hour. You can also search for language partners and post your writing for correction (for free).

Word Reference: http://www.wordreference.com/
-- Good online dictionary. Has conjugations and many expressions and collocations in addition to single words (also useful for other languages).

For intermediate and above

RFI - Journal en français facile: https://savoirs.rfi.fr/fr/apprendre-enseigner/langue-francaise/journal-en-francais-facile
-- News broadcast in somewhat simplified French with a transcript

The "Grammaire progressive" and "Vocabulaire progressif" series from CLE: https://www.cle-international.com/actualites/grammaire-progressive-du-francais.html
-- Excellent exercise books (in French).

Banque de dépannage linguistique: http://bdl.oqlf.gouv.qc.ca/bdl/listearticles.aspx
-- A resource maintained by the Office Québécois de la Langue Française. Many articles (in French) about vocabulary, points of grammar, anglicisms.

Le Wiktionnaire: https://fr.wiktionary.org/wiki/Wiktionnaire:Page_d%E2%80%99accueil
-- Crowdsourced online dictionary (in French). Good source for recent and colloquial expressions that other dictionaries may not have.