Ann Arbor has a car sharing co-op but it’s not the most user-friendly organization. That’s why I was pleased when I found out that Zipcar is available in Ann Arbor just like it is in Toronto, Chicago, Vancouver, and San Francisco. They have locations convenient to people on central campus (University of Michigan of course), the U of M medical center, and North Campus. They just need to add a location on the Old West Side so those folks can start helping the city become less auto-dependent.
Detroit also has a car sharing co-op, Motor City Car Share, but it’s currently on hiatus from accepting new members. But I would still recommend contacting them if you live in Detroit and are interested.