A online resource which I had not noticed before - Zeitungen des 17. Jahrhunderts
Despite the title there are a lot of 18th century material as well, including issues of Stralsundischer Relations Corier, Revalische Post-Zeitung and Rigische Novellen. Very useful for tracing how news and rumours travelled across Europe.
A Swedish version covering some of the major domestic 19th and 20th century papers - http://tidningar.kb.se/. It's usefulness is still somewhat limited due to an extremely careful handling of the copyright issue (only items older than 1901 can be read online).
An older attempt at something similar, but this mostly with smaller local papers - http://magasin.kb.se:8080/searchinterface/
And a Dutch version going back to the 17th century: http://www.delpher.nl/