Galleycat has a list of 25 horror ebooks that you can get for free over at Project Gutenberg. A couple highlights:

Frankenstein. Mary Shelley’s novel still might be the best horror story ever told.

Dracula. Bram Stoker’s classic is a portrait of late-Victorian paranoias about sexuality and foreigners—but it’s still pretty chilling.

The Strange Case of Dr. Jekyll and Mr. Hyde. This novella by Robert Louis Stevenson has elements of a mystery and detective novel, but ultimately it’s a horror tale and a sly metaphor for the double lives of men in late-Victorian London.

The Mysteries of Udolpho. The quintessential Gothic novel, and a major inspiration for Jane Austen’s Northanger Abbey.

Wieland. Forget Grant Wood—this is the real American Gothic, and one of the first-ever American novels.

The Beetle. This novel by Richard Marsh is similar in some ways to Dracula.

Curious, if True. A collection of Victorian ghost stories by Elizabeth Gaskell.

I just downloaded a bunch of these. Perfect for Halloween reading!