For the state of Michigan that is available at Michigan Online Historical Newspapers Summary.
My updates are semi-annual or annual, depending on how much has been changed. For example, my most recent Michigan update was in November, 2014.
I want to make you aware of a terrific resource for Michigan historic newspapers that can help you.
Central Michigan University has created an outstanding resource that is more frequently updated and has some nice features that can help you hone in on your county or newspaper of choice. It also has a very useful map of collections and where they are hosted .And that is the Michigan Digital Newspaper Portal. It provides a list of the online historical newspapers for the state available in a variety of collections (current total of titles about 250).
It lists available newspapers and their collection links alphabetically by county, and also indicates whether the collection is available for free or via a subscription
Furthermore, you can access the Library of Michigan's "family history" of a newspaper, as it's title and frequency may have changed over the years. This is displayed graphically, which is very easy to read. This is quite helpful.
The Michigan Digital Newspaper Portal from CMU is an outstanding resource for finding articles about your Michigan ancestors. I heartily encourage you to make use of this wonderful site.
For free links for other states, please check out the Newspapers! Page on The Ancestor Hunt site.