Newspapers.com is one of the largest online archives of historical newspapers, focused mainly on the United States and Canada, but also from other countries.
What it offers:

• Access to hundreds of millions of scanned newspaper pages from the 18th century to the present.
• Search by name, place, date or keyword - widely used for genealogy (e.g. searching for death notices, weddings, articles about ancestors).
• Full-text search.
• The newspapers are of very good quality - full pages with advertisements, headlines, articles, photos, etc.

Cons:
It is not free. It works on a subscription basis, although some parts are accessible without registration.
Who is it suitable for:
Historians, genealogists, students, journalists or even writers looking for period realities, people looking for mentions of family, events in the city, school anniversaries, sports results or even just retro advertisements.