Adjectives that look like adverbs

beastly, cowardly, deathly, fatherly, friendly, princely, womanly
taken from: Huddlestone and Pullum's grammar

lonely
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"