As indicated in a previous answer, most coffee shops and many restaurants in Ann Arbor have "free" WiFi, though as we all know they aren't really free as there is some expectation of an accompanying purchase. The downtown public library has truly free WiFi. read more
For example, you can sit in Qdoba and connect to the S. Main Expresso Royale access point. Local wifi maps. a2wifi has a list of wireless locations in Ann Arbor and Dexter. The University of Michigan has wireless access in some parts of campus; there is a somewhat baroque interactive map of these locations. read more
Alternately, there's a cooperative of users in San Francisco, called Free-the-Net, which is community based. Internet subscribers are eligible for a free wifi router if they provide their neighbors with free access. That program works very well in a densely populated city like SF. Ann Arbor residents should do the same. read more