What you have appears to be a Round Black Spanish radish that's gone to seed. They're humongous storage radishes with a black skin, and can be eaten raw or cooked. I grew some a few years ago, and contrary to what you'd expect, they weren't too tough.

The round ones may reach 4-5" in diameter. There are also Long Black Spanish radishes, which are pointed or cylindical and not so wide.

You could cut the outer edges off and try to eat the round one; I've no idea if it would be woody or tough at this time of year.

You can also save some seeds and grow a few this spring. Or buy seeds from Baker Creek, Johnny's, Burpee, Seeds of Change, Pinetree Garden, Southern Exposure Seed Exchange, etc.

Lots of background here:
http://www.specialtyproduce.com/produce/Black_Radish_752.php

That's superb, thanks!

I bet the skin is really good for you too, as a general rule, like all other tubers tend to be.

Yep the four petaled flowers say mustard famiy and the seed pods look like a radish, too. Up here they are called winter radishes, planted in late summer and harvested as late in the autumn as possible. Normally then they do not go to seed, like those planted in the spring do.

There are a couple of types of Black Spanish, a couple of types of German, which are a bit milder and a number of very similar Oriental types like the daikons, which are my favorite, and generally quite mild. The Spanish types tend to be hot. The hybrid daikons, like Spring Cross and Summer Cross can be spring planted and will produce long white roots 2-4" x as much as 18 or 20" long, sometimes with a green crown.

Most supermarkets carry them if you would like to try them first. They last quite well and stay nice and crisp in the frig. Try them as sticks raw perhaps in combination with sticks of seedless greenhouse cukes, or a couple of slabs lightly salted topped by a slices of beefsteak tomato to make an excellent two bread slice sandwich. My family has always liked radish sandwiches, even made out of the standard small spring radishes.

They are easy to grow but the daikon types take some digging to harvest, since they seem to go down and down and down; so be sure to dig your beds for them deep so the roots have depth to expand down into.

Green radish seed pods are also edible, but tend to have a pretty decent bite to them. Good with a cold beer.

Some of the rooted members of the brassica group are commonly grown as deer food plots and/or as garden cover crop which is also often grazed by organic farmers who also raise meat animals. They all do best in cooler weather.