Creativewritingprompts.com
Appears to be a good site. Includes good writing prompts, software, e-mail courses, and newsletters. I'm going to take the free course on imagery.
The writing prompts appear to be stored in a BLOG. I had a similar idea myself of using a BLOG in this way with prompts for science fiction, horror, and fantasy writers. It appears to me, there are not a lot of writing sites or magazines directed toward that market. File that idea under "future."