Not sure if I'd call it a killer scarecrow movie, but I voted Jeepers Creepers.
Somewhat related, but not a movie, there's a terrific episode of the TV series Are You Afraid Of The Dark? titled "The Tale Of The Silent Servant" that is worth seeking out for fans of the evil scarecrow genre.
There's also the fairly entertaining straight-to-video slasher flick Slash, and the terrible Full Moon flick Dark Walker. Even though Dark Walker is a dud, it's still better than LionsGate's Dark Harvest series.