For a month or two in spring, you can get fresh stinging nettles. If you know how to look for them, you can forage for them around your neck of the woods -- if not, there are various places online where you can buy them. (Check Localharvest, for instance, or Marx Foods.)
If you buy them, they won't be cheap, partly because they're a pain in the ass to harvest -- there's a reason they call them stinging nettles. They're covered with little hairs, and each hair has a tip that falls off when touched, turning the hair into a needle that pricks you with a number of chemicals, depending on the species -- some nettles even hit you with formic acid, the same vicious little concoction that fire ants have. This sucks. This hurts. It can last a few minutes, it can last a few weeks. Your hand can swell up like a baseball glove. It all depends on the species.
But the chemical is neutralized by cooking. Very young nettles can be eaten raw, but having been stung by nettles, I can't see taking that risk. Cooked, they're a bit like spinach -- but while most spinach-like wild greens can have a somewhat swampy flavor, nettles are full-flavored but very accessible and user-friendly. If you told your kids or spouse or whatever that it was spinach, they wouldn't give you any guff.
All you need to do is put a little water in a pot, bring it to boil, and dump the nettles in until they're obviously cooked. A little longer than blanching, to be safe.
They stay very green after cooking. Even the pot liquor is dark green:
Look at that. Think of how fucking healthy that is!
They're apparently extremely high in protein. The dried leaves are used to make tea, and a cordial used to make soda is popular in parts of Europe -- I haven't tried that yet.
In American restaurants, you're likeliest to see nettles cooked in Italian contexts -- Alice Waters' joint has served nettles pizza for a while, and nettles pesto is around, or nettles with polenta, that kind of thing.
I was very happy with nettles "champ." After steaming, I squeezed the liquid out of the nettles, chopped them up, and added them to a pan with mashed potatoes and butter. Served it with smoked leg of lamb and miso gravy (which had actually been made for a pulled pork and mustard greens sandwich: roast pork fond and stock and maitake mushrooms simmered for a while, strained, with red miso whisked in).
Matt and I had a nettles stuffed pizza that came out very good: I put nettles, feta, mozzarella, and a shake of crushed red peppers on half the crust, folded it over and pushed the air out, let it rise half an hour, and then put sauce and cheese on top and blitzed it in the oven per usual. Very nice.