Orthogram.com: Learn French adverbs. Contains a short explanation of 6 types of French adverbs. Adverbs of manner, adverbs of frequency and quantity, adverbs of time, affirmative adverbs, negative adverbs, and adverbs of doubt.
Synapse-fr.com: Free online French grammar theory manual that explains the elements of the French language. Type of words in a sentence. Negative adverbs.