Believe it or not this appears to be an Eastern garter snake. Though there is some variation in the coloring and pattern in most snakes as they age, the Eastern garter snake has a fairly distinct checkered pattern on it's back. The only other snake found in Michigan that is really similarin appearance is the Northern water snake, although that is usually thicker bodied and, like the name implies, usually is found near water.
The Michigan Department of Natural Resources has a nice online resource for identifying snakes found within the state. You can access the site here:http://www.michigan.gov/dnr/0,1607,7-153-10370_12145_12201-61219--,00.html#Species and see the entry on Eastern garter snake here: http://www.michigan.gov/dnr/0,1607,7-153-10370_12145_12201-61200--,00.html