In Norway these little piles of stones on top of mountains are called Trolls. Whoever climbs a mountain usually leaves one there, according to the local custom. It's believed that at nights the Trolls come alive, dance on the mountains and do whatever else Trolls are supposed to do, only to turn to stone again with coming of sunrise.

EDIT: a couple of corrections to my story:

by ~idnar88
I thought I should tell u that piles of stone like that is called a 'Varde' where I come from in Norway and according to my historybook its was built to keep the trolls away when people camped for the night, it was supposed to scre them away.. but ur version might be right also, many versions of these things.

by ~2473
Even though some trolls are rumored to be made out of stone, they don't really come back to life at night, they just die and turn into stone when the sun shines on them. There are tales about whole mountains once being trolls that somehow didn't manage to hide before the sun rose and killed them, but none are as small as that, or built up of separate stones. Some mountains or small islands are even supposed to have been objects that trolls have lost and that also have turned into stone. The reason for these tales are that when people in earlier times didn't have science to rely on, they'd make up tales to explain the reason for things they wondered about.

by ~Valantar
What your picture shows is called a 'varde', it is sort of a beacon (like the ones between Gondor and Rohan in The Lord of the Rings). They were used as signaling systems (people lit a fire there, and it was visible for miles and miles) back in earlier stages of society, and there has never been anything mythical about them.

Other people also reported similar traditions in UK, Canada, Spain and Switzerland...