Elephind.com makes the claim to search all of the world's digital newspapers at one time. This may be an overstatement, but it does have a search engine that will search 141,628,238 items from 2,677 newspaper titles. Its collections are decidedly slanted to newspapers from Australia and New Zealand but it does have papers from places as diverse as Singapore and Mexico. Part of the huge collection comes from a link to the National Library of Australia's website Trove.nla.gov.au. Another part is linked to the Library of Congress's Chronicling America website.
Here is a screenshot of the top of the current titles page:
As with any large online database, if you find your ancestors you think it is great. If you don't find your ancestors, you don't think it is worth much. But like everything in genealogy; the game is in the search.