Harvard University's Berkman Center has launched an Online Media Legal Network, described as "a network of law firms, law school clinics, and individual lawyers across the United States who are interested in assisting online journalism ventures and other digital media creators address their legal needs." Some may provide free or reduced fee assistance.
Membership organizations such as The American Society of Journalists and Authors, The Author's Guild, and the National Writers Union offer some useful legal information to website visitors and many more benefits, including various legal services, to those who become members. There are some eligibility requirements for membership.
A more specialized resource is the Reporters Committee for the Freedom of the Press aimed, as the name would suggest, at journalists. The RCFP maintains a legal defense hotline for professional reporters and media lawyers dealing with legal emergencies at 800-336-4243. (Their services are not directed toward literary writers.)
Two other organizations that do much good work for writers are Poets & Writers, Inc. and PEN American Center, albeit I frankly do not know whether they provide referrals for volunteer legal services.
If you know of other organizations that belong on this list, please post a comment to let me know.