I LOVED The Little Old Woman Who Was Not Afraid of Anything when my kid was younger -- it walks the line of "spooky, but not TOO scary for little kids" so nicely! Also, for 4-6 year olds, the Georgie series by Robert Bright are lovely -- he's a shy little ghost who haunts a house in an idealized New England town.
My nine-year-old picked up "Short and Shivery" by Robert San-Souci this weekend at the library -- she says it's not scary at all, scoff scoff, but I've noticed she can't put it down. It's all retellings of stories from various folklore and urban legends - the tailypo, vanishing hitchiker, etc. There's a good African one about a were-hyena!