Internationally-known deli. Sandwiches, bread, cheese, deli, olive oil, vinegar, various fancy and tasty things to eat. They make their own bread and some dairy products. Prices are high, but so is quality and quantity (if you get a large sandwich, plan on either splitting it with someone or saving half of it for your next meal).
Don't miss their brownies (if you only try one kind, try the original ones with nuts), and don't hesitate to ask for samples of anything that looks interesting, including the 80-year-old balsamic vinegar. Money-saving tips: day-old loaves of bread are available at a steep discount, and all breakfast pastries become half off after 5pm.
The first member of the Zingerman's Family of Businesses, Zingerman's Deli was founded by Ari Weinzweig and Paul Saginaw in 1982. (And no, there isn't actually a "Zingerman." The name just sounded cool, and wasn't already taken.)
422 Detroit Street (at Kingsley Street)