The Sunroom is pretty close to Madison's best American style diner. It looks like a hole in the wall and you have to go up a flight of stairs to get to it, but Sunroom's dining room is surprisingly charming, albeit without a view of the sun.
The menu is vast and they serve breakfast through 2:00pm. I've only tried their breakfast, but their omelettes, coffee and breakfast potatoes are among the best I've come across. Prices are reasonable and the food is non-greasy. I've heard good things about their pastries and smoothies. I'll be back.