There is no way on earth this is going to be complete, but here's something like a list. comments welcomed. here is an NPR story and interview with some background; some places require indication of voting, others will go on the honor system.
(didn't find anything special yet for Ann Arbor, but I'm sure there's something)
Krispy Kreme: free donut (via Budget Travel / Newsweek / New York)
Ben and Jerry's: free scoops 5p-8p (via New York)
Starbucks: free tall coffee (via srah)
SitterCity: free or discounted babysitting (new subscribers only)
CREDO Mobile: free calls