Public Citizen has helped numerous people keep their free speech rights:
http://www.citizen.org/litigation/briefs/IntFreeSpch/articles.cfm?ID=5799

The Wikipedia article about gripe sites:
http://en.wikipedia.org/wiki/Gripe_site

Another list of gripe sites (along with some helpful information about starting your own):
http://www.webgripesites.com/

An interesting piece about how at least some lawyers are realizing the pointlessness of threatening or suing the operators of gripe sites, noting that most of them are completely legal:
http://www.techdirt.com/articles/20090405/1832594397.shtml

From the other side of the fence, a company trying to sell a service to shut down gripe sites (worth a read):
http://www.hostingdude.com/internet_complaint_sites/stop_internet_complaint_sites.htm

Another interesting article about gripe sites in general:
http://www.associatedcontent.com/article/60685/gripe_sites_when_something_sucks_post.html