An article from the Idaho Observer says,
Nettles are used to increase fertility in both men and women. Due to its high calcium content, the tea is specific for easing leg cramps and other muscles spasms, and also diminishes pain during and after birth.It does a lot of other stuff that you can read about here.
Most people need gloves to harvest them, by the way. And only use them before they flower; once they go to seed they are hard on the kidneys. Also, many people consider them to be a weed, so be careful where you get them and make sure they aren't being sprayed with pesticide! You can often also buy nettle seeds and grow your own.
Nettles also helped me get rid of my seasonal allergies. Read here.