Press Release Marketing is the process of publishing a press release, for the purposes of generating additional traffic to a website.
PR News Wire and PR Web are two important ones, as they can both feed online news search engines. Prweb no longer has a free option. PRLEAP still does, but is extreeeeemely tough on the format of press releases submitted.
SEO PR is a company dedicated to marketing in this method.
Some popular PR sites include: For reference, here is a list of additional press release sites: