What is Bitcoin-Crane?

Often we hear the question - how to get bitcoin for free? Although they say, but make some efforts still have to. There are bitcoin-cranes, by spending some time, you get cryptocurrency on your wallet.
Basically, these sites are advertising. The entrance to such resources is usually free, but sometimes you need to pass a simple registration procedure. Most often, for registration you only need to specify the email address and your wallet address - wait a bit and click - get Satoshi.