For beginners

Assimil French:
-- The book + CD pack that I used to start learning French. Well worth the price.

French In Action:
-- A video series teaching French by “immersion” (no translation) from the very basics to intermediate level.

Duolingo French:
-- French learning site / mobile app.

-- 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:
-- 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:
-- News broadcast in somewhat simplified French with a transcript

The "Grammaire progressive" and "Vocabulaire progressif" series from CLE:
-- Excellent exercise books (in French).

Banque de dépannage linguistique:
-- A resource maintained by the Office Québécois de la Langue Française. Many articles (in French) about vocabulary, points of grammar, anglicisms.

Le Wiktionnaire:
-- Crowdsourced online dictionary (in French). Good source for recent and colloquial expressions that other dictionaries may not have.