A site called Elephind.com — it searches free, online newspaper content. Not the subscriptions, but those disperse digital collections that are available from a wide variety of providers.
It searches almost 2,000 titles from 5 countries and 15 websites. That is NOT all the free newspaper content on the web. For example, it doesn’t search the wonderful Fulton History site, which contains over 26 million pages of historical New York newspapers. So don’t count on it to catch everything.
The site says they are adding newspapers everyday and it is very clearly laid out, so it is easy to tell what sites are being searched and from there, to find lists of newspapers being searched.