Although not devoted exclusively to newspaper articles, both of these databases offer a healthy number.
Alt-PressWatch (Proquest) : Full-text database of selected newspapers, magazines and journals of the alternative and independent press. Coverage complements the reporting in the mainstream press.
Ethnic NewsWatch (Proquest) : The Ethnic NewsWatch database provides the full text of ethnic and minority newspapers, magazines, and journals in the United States. Coverage varies by paper, but goes as far back as 1960 for some publications.