Adjectives that look like adverbs
beastly, cowardly, deathly, fatherly, friendly, princely, womanly
taken from: Huddlestone and Pullum's grammar
from Wikipedia: Flat adverb
deadly, lowly, manly, slovenly
(things we came up with ourselves :))
"Friendly" seems to be a difficult case - some people regard it as "adjective only",
others as "adjective and flat adverb"