The Michigan State Archives, somewhat mysteriously housed on the Department of Natural Resources website, is nonetheless, a rich collection for history buffs and a pretty good resource for genealogists.
The best of Michigan's digital archives do claim their own website, though, called Seeking Michigan. The site is heavy on Civil War materials. These are browseable, but there's no name lookup capability for most of the collections:
There's a lot of additional materials, mostly from the 19th and 20th centuries:
and of course, the ever-popular... Governors of Michigan
There is also a series of articles on a variety of historical topics, chiefly focused on notable Michiganers (Michiganites? Michigonians?).
Back at the DNR State Archives page you'll find a few more items of interest:
And a few more odds and ends scattered about: