Dropping L in compound adjectives. Is it "skillful" or "skilful"?
It's largely an American vs British spelling difference, with Americans preferring skillful.
(Note that the British version of Macmillan says skillful is the "American spelling of skilful.")
As Gnawme states, "skillful" is an American English spelling. However, "skilful" is not necessarily just British English :
For the adjective meaning "with skill or having skill", "skillful" is the preferred spelling in American English. "Skilful" is preferred in all other varieties of English.