Michigan Department of History, Arts, and Libraries
HAL has a lot of different kinds of maps and map sources

Michigan Department of Information Technology
Center for Geographic Information  - CGI has a lot of different maps and information.  Some of them are: Parks and Recreation, Census, Political Boundary, Digital Ortho-photo, School Districts, and Natural Resources.
Oil and Gas Maps by County  select a county and get a map in .pdf format.  By using the magnifying tool in Acrobat you can get permit numbers and other information from the map.
Soil maps and other data

Michigan Department of Labor and Economic Growth
Michigan Public Service Commission - oil and gas pipeline maps, electrical maps

Michigan Department of Natural Resources
The DNR has a wide variety of maps.  Some examples are: Wildlife, Fishing, Recreation, Coal Mines, Land Office Plats, Watershed, Trails, Counties, and Rivers.

Michigan Department of Transportation  
The DOT has road maps, bicycle paths, traffic counts and other maps.

Michigan State University
Center for Remote Sensing - aerial photos and other information
Map Library - atlases and various other maps

US Department of Agriculture, Natural Resources Conservation Service
Michigan Office or National Site
This federal agency has information about plants, animals, soils, water, and air.
Michigan soil maps,  National soil data

US Geological Survey
The USGS has different maps from across Michigan and the whole United States, including digital elevation models, orthophoto quadrangles, topo, rivers, land cover, and census.
Earth Science Information Center - sales and service for all USGS maps and Publications.