These are Collard Greens. They're pretty popular down here in the South. They're eaten year 'round, but are best in the winter, because they store a lot of nutrients in their leaves for the winter. The leaves are slightly bitter, which I believe is why I've always gotten the impression they're more of a lower-class food; why go for nutritious when you can go for tasty?
This is Kale. Kale used to be one of the most popular in Europe. Like collards, they're grown world-wide and respond well to freezing, actually tasting better afterward. Because of its striking coloration, it even comes in ornamental varieties.
This is Cabbage. It's got a lot of leaves packed into one spot, is a good source of vitamin C, and is naturally a little spicy. This makes it quite popular in a variety of dishes.
These are Brussels Sprouts. They get a pretty bad rap, probably because if you overcook them they end up grey, mushy, and bad-tasting. But if you cook them right, they're flavorful and might fight cancer! Either way, they're full of good nutrients, like tiny cabbages!
This is Romanesco Broccoli. Look at it. Just look at it! It's a frickin' fractal! It's a logarithmic spiral of buds, each of which is a logarithmic spiral of buds, each of which is a logarithmic spiral... Just look at it!
This is Broccoli. The first President Bush didn't like broccoli. He can go right to hell. Broccoli is awesome.
This is Cauliflower. It looks like albino broccoli, doesn't it? Don't let that weird you out. It's different, but it's pretty awesome. Low in fat, low in carbs, high in fiber and nutrients. It's a superfood! Also, tasty.
This is Kohlrabi. It's another guy that's popular in Europe, but never really made it to the US. Apparently Charlemagne ordered them grown throughout his kingdom. It's name means cabbageturnip. It does look an awful lot like a turnip.
So what do all of these have in common? These guys.
That's wild cabbage and wild mustard, of the brassica family. Cabbage has been domesticated more than half a dozen different ways, focusing on different aspects. Its leaves, its roots, its flowers... and all because it's super-nutritious and a little bit spicy! One: Cabbage is awesome. Two: So are we, for figuring that out. Three: Us again, because we've come up with so many ways to take advantage of cabbage's cabbageosity.
Remember turnips? Well, there's a reason kholrabi look like turnips; turnips are a closely related species of brassica! They're cousins! Turnips are pretty cool, too. The root's got some vitamin C. The leaves, for which they've also been cultivated, have a whole other mess of crap, too, like folate and fiber and junk. Go, turnips!
Beets are another variety of plant that have been cultivated multiple times. Sugar beets, chard, beetroot... Man, farming's pretty darn creative.