Legislative Branch: The Michigan Legislative Information Service provides Bills, Bill Status information, Acts, the Michigan statutory code, the Michigan Constitution, and an Archive with prior editions of the Constitution. For commercial database vendors, see State Statutes and Codes and State Bills. For legislative history guides, see State Legislative History.
Judicial Branch: Reported cases are available free back to 1950 from Google Scholar. Lexis and Westlaw provide comprehensive case databases. For additional resources, see State Cases, Case Pulls, Docket Sheets and/or State Court Rules.
For more information: Links to primary legal materials and government agencies are posted by FindLaw and WashLaw.
Michigan Legal Literature: An Annotated Guide is still historically useful but, to my knowledge, was last published in 1991.
For questions about Michigan legal materials, copies and/or inter-library loan, try the Michigan State University College of Law Library (517-432-6870) or the University of Michigan - for document delivery or loans call MITS (734-763-5060), for questions call the University's Law Library (734-764-9324).
Business entities: You can look up Michigan business registrations using the Corporation Division Business Entities Search. For questions, call the Corporation Division at 517-241-6470.