This is Boston Brown Bread. It's made commercially by B&M (the baked bean people) and in New England, you can often find it in the supermarket right alongside the baked beans. Although it looks vaguely turd-like, it's quite delicious, and makes an excellent accompaniment to baked beans. It's made with flour, cornmeal, and molasses and it has a fairly dense texture, more cake-like than bready.
The best way to serve it is heated (either in the microwave or the oven), then sliced, spread with butter, and put on the plate right next to a big scoop of beans & weenies as shown at left. When I was a kid, my mother rarely served baked beans without B&M Brown Bread alongside. And I am happy to report that it is one of the very few food products from my childhood that still tastes exactly the same today as it did forty-some-odd years ago. (Yes. Holy shit. I really am that old.)
I've always taken for granted that B&M Brown Bread is available in the stores, though I know that it's probably one of those New England things that can be hard to find once you get out of the Northeast (kind of like Bell's Poultry Seasoning.) But if you're really interested in trying it, you could make your own with Alton Brown's recipe from Food Network.