sticker image from electionstickers.com
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.
Local:
(didn't find anything special yet for Ann Arbor, but I'm sure there's something)
National:
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)
Regional:
Austin, TX: free bus rides on Capital Metro; free cab rides for seniors and the mobility impaired
Chicago, IL: free coffee from Metropolis Coffee
Fargo, ND: free bus and paratransit rides
Random:
CREDO Mobile: free calls
Edward Vielmetti's Wordpress weblog 
I thought you were buying ?
Does anyone know of an “I voted absentee!” sticker? I’m gonna feel left out on Tuesday without my sticker,but my ballot has been mailed in for weeks.
i heard a rumor in the voting line that sidetracks bar & grill in depot town, ypsi is giving away a free beer today if you have a coupon and an i voted sticker.
confirmed by steve krause’s blog.
check it:
http://emutalk.org/?p=1201
Free admission at the Michigan Theater tonight. They’ll be putting up election results on the screens in the lobby, too.