Newspaper Web Editions
For a fairly comprehensive Listing of all web editions of newspapers, check online newspapers.com.
Newspaper Sources by polity
- United Kingdom
- United States
- Germany
- Austria
- Ireland
- Finland
- European Union
Databases
- LexisNexis
LexisNexis is undoubtedly the most extensive database on newspaper data, it contains full text articles of all major US and European papers as well as most local US papers. Unfortunately, unless your institution is a subscriber, access to LexisNexis is very cost intensive (around US-$ 5 per retrieved document). 
Links to Archives of Newspapers
(worldwide)
Paperball
Paperball is a search machine for German News. It searches all major national and local papers. Unfortunately, you can only search the news of the current day.