The venerable Saugus.net Halloween Ghost Story Writing Contest is open for submissions in its twentieth year. It's completely free to enter, and Saugus.net is a non-profit organization that places no restrictions on authors' future use of entries. It is open to adults and children (kids have their own categories: high school, middle school, and elementary school levels so they're not competing against the adults) and open to published and unpublished authors. It also accepts both prose and interactive fiction offerings.
The deadline for entering is Oct. 22nd, and winning entries are posted live on Halloween night, Oct. 31. Winning entries from all past nineteen years can also still be read online, too.