Fiction Factor: The authors state that their listings "are by no means exhaustive" but you can get exhausted looking through all these links—all free.
Forwriters.com: Not a database, but an extensive list of links to writer's guidelines located at an individual magazine, e-zine or book publisher's Web site.
Freelancewriting.com: Over 1,000 writer's guidelines in their database.
Writing For Dollars: Hundreds of writer's guidelines in this searchable database with new ones added every week.
WritersWeekly.com Freelance Jobs and New Paying Markets: Market listings are created from interviews with editors and are based on the publication's current needs. Also includes a link to markets that welcome new writers.
Writer's Write Guidelines Directory: Extensive listing, completely searchable of writer's guidelines on the Web.